Envelope

Quick layer over python-gnupg, M2Crypto, smtplib, magic and email handling packages. Their common usecases merged into a single function. Want to sign a text and tired of forgetting how to do it right? You do not need to know everything about GPG or S/MIME, you do not have to bother with importing keys. Do not hassle with reconnecting SMTP server. Do not study various headers meanings to let your users unsubscribe via a URL.
You insert a message and attachments and receive signed and/or encrypted output to the file or to your recipients' e-mail.
Just single line of code. With the great help of the examples below.

Envelope("my message")
    .subject("hello world")
    .to("example@example.com")
    .attach(file_contents, filename="attached-file.txt")
    .smtp("localhost", 587, "user", "pass", "starttls")
    .signature()
    .send()

• Installation
  • Bash completion
• Usage
  • CLI
  • Module: one-liner function
  • Module: fluent interface
• Documentation
  • Command list
    • Input / Output
    • Cipher standard method
    • Signing
    • Encrypting
    • Sending
      • Specific headers
    • Supportive
  • Default values
  • Converting object to str or bool
• Examples
  • Signing and encrypting
  • Sending
  • Attachment
  • Complex example
• Related affairs
  • Configure your SMTP
  • Choose ciphering method
    • Configure your GPG
    • Configure your S/MIME
  • DNS validation tools
    • SPF
    • DKIM
    • DMARC

Installation

• If planning to use S/MIME, you should ensure some prerequisites:

sudo apt install swig
pip3 install M2Crypto

• Install with a single command from PyPi

  pip3 install envelope
  

  • Or install current GitHub master

  pip3 install git+https://github.com/CZ-NIC/envelope.git
  

  • Or just download the project and launch ./envelope.py
• If planning to send e-mails, prepare SMTP credentials or visit Configure your SMTP tutorial.
• If your e-mails are to be received outside your local domain, visit DMARC section.
• If planning to sign/encrypt with GPG, install the corresponding package and possibly see Configure your GPG tutorial.

sudo apt install gpg

Bash completion

1. Run: apt-get install bash-completion jq
2. Copy: extra/convey-autocompletion.bash to /etc/bash_completion.d/
3. Restart terminal

Usage

As an example, let's produce in three equal ways an output_file with the GPG-encrypted "Hello world" content.

CLI

Launch as a CLI application in terminal, see envelope --help

envelope --message "Hello world" \
               --output "/tmp/output_file" \
               --sender "me@example.com" \
               --to "remote_person@example.com" \
               --encrypt-path "/tmp/remote_key.asc"

Module: fluent interface

Comfortable way to create the structure if your IDE supports autocompletion.

from envelope import Envelope
Envelope().message("Hello world")\
    .output("/tmp/output_file")\
    .sender("me@example.com")\
    .to("remote_person@example.com")\
    .encrypt(key_path="/tmp/remote_key.asc")

Module: one-liner function

You can easily write a one-liner function that encrypts your code or sends an e-mail from within your application when imported as a module. See pydoc3 envelope or documentation below.

from envelope import Envelope
Envelope(message="Hello world",
        output="/tmp/output_file",
        sender="me@example.com",
        to="remote_person@example.com",
        encrypt="/tmp/remote_key.asc")

Documentation

Both envelope --help for CLI arguments help and pydoc3 envelope to see module arguments help should contain same information as here.

Command list

All parameters are optional.

• --param is used in CLI
• .param(value) denotes a positional argument
• .param(value=) denotes a keyword argument
• Envelope(param=) is a one-liner argument

Any attainable contents means plain text, bytes or stream (ex: from open()). In module interface, you may use Path object to the file. In CLI interface, additional flags are provided.

Input / Output

• message: Message / body text. If no string is set, message gets read. Besides, when "Content-Transfer-Encoding" is set to "base64" or "quoted-printable", it gets decoded (useful when quickly reading an EML file content cat file.eml | envelope --message).

  • --message: String
  • --input: (CLI only) Path to the message file. (Alternative to --message parameter.)
  • .message(text): String or stream.
  • .message(path=None): Path to the file.
  • .body: Alias of .message
  • .text: Alias of .message
  • Envelope(message=): Any attainable contents

  Equivalents for setting a string (in Python and in Bash).

  Envelope(message="hello") == Envelope().message("hello")
  

  envelope --message "hello"
  

  Equivalents for setting contents of a file (in Python and in Bash).

  from pathlib import Path
  Envelope(message=Path("file.txt")) == Envelope(message=open("file.txt")) == Envelope.message(path="file.txt") 
  

  envelope --input file.txt
  

• output: Path to file to be written to (else the contents is returned).

  • --output
  • .output(output_file)
  • Envelope(output=)

Cipher standard method

Note that if neither gpg nor smime is specified, we try to determine the method automatically.

• gpg: True to prefer GPG over S/MIME or home path to GNUPG rings (otherwise default ~/.gnupg is used)
  • --gpg [path]
  • .gpg(gnugp_home=True)
  • Envelope(gpg=True)
• .smime: Prefer S/MIME over GPG
  • --smime
  • .smime()
  • Envelope(smime=True)

Signing

• sign: Sign the message.
  • key parameter
    • GPG:
      • Blank (CLI) or True (module) for user default key
      • key ID/fingerprint
      • Any attainable contents with the key to be signed with (will be imported into keyring)
      • "auto" for turning on signing if there is a key matching to the "from" header
    • S/MIME: Any attainable contents with key to be signed with. May contain signing certificate as well.
  • --sign key: (for key see above)
  • --sign-path: Filename with the sender's private key. (Alternative to sign parameter.)
  • --passphrase: Passphrase to the key if needed.
  • --attach-key: GPG: Blank for appending public key to the attachments when sending.
  • --cert: S/MIME: Certificate contents if not included in the key.
  • --cert-path: S/MIME: Filename with the sender's private cert if cert not included in the key. (Alternative to cert parameter.)
  • .sign(key=True, passphrase=, attach_key=False, cert=None, key_path=None): Sign now (and you may specify the parameters). (For key see above.)
  • .signature(key=True, passphrase=, attach_key=False, cert=None, key_path=None): Sign later (when launched with .sign(), .encrypt() or .send() functions
  • Envelope(sign=key): (for key see above)
  • Envelope(passphrase=): Passphrase to the key if needed.
  • Envelope(attach_key=): GPG: Append public key to the attachments when sending.
  • Envelope(cert=): S/MIME: Any attainable contents.

Encrypting

If the GPG encryption fails, it tries to determine which recipient misses the key.

• encrypt: Recipient GPG public key or S/MIME certificate to be encrypted with.
  • key parameter
    • GPG:
      • Blank (CLI) or True (module) to force encrypt
      • key ID/fingerprint
      • Any attainable contents with the key to be encrypted with (will be imported into keyring)
      • "auto" for turning on encrypting if there is a matching key for every recipient
    • S/MIME any attainable contents with certificate to be encrypted with or their list
  • --encrypt key: (for key see above) Put 0/false/no to disable encrypt-path.
  • --encrypt-path (CLI only): Filename(s) with the recipient's public key. (Alternative to encrypt parameter.)
  • .encrypt(key=True, sign=, key_path=):
    • sign You may specify boolean or default signing key ID/fingerprint or "auto" for GPG or any attainable contents with S/MIME key + signing certificate.
    • key_path: Key/certificate contents (alternative to key parameter)
  • .encryption(key=True, key_path=): Encrypt later (when launched with .sign(), .encrypt() or .send() functions. If needed, in the parameters specify Any attainable contents with GPG encryption key or S/MIME encryption certificate.

  # message gets encrypted for multiple certificates
  envelope --smime --encrypt-path recipient1.pem recipient2.pem --message "Hello"
  

  • Envelope(encrypt=): Any attainable contents
• to: E-mail or list. When encrypting, we use keys of these identities. Multiple addresses may be given in a string, delimited by comma (or semicolon). (The same is valid for to, cc, bcc and reply-to.)
  • --to: One or more e-mail addresses.

    envelope --to first@example.com second@example.com --message "hello" 
    

  • .to(email_or_list): If None, current list is returned.

      Envelope()
          .to("person1@example.com")
          .to("person1@example.com, John <person2@example.com>")
          .to(["person3@example.com"])
          .to()  # ["person1@example.com", "John <person2@example.com>", "person3@example.com"] 
      ```
  * **Envelope(to=)**: E-mail or their list.
  

• from: E-mail – needed to choose our key if encrypting.
  • --from E-mail
  • --sender Alias for --from if not set. Otherwise appends header "Sender".
  • --no-sender Declare we want to encrypt and never decrypt back.
  • .from_(email): E-mail or False. If None, current From returned.
  • .sender(email): E-mail or False – an alias for .from_, will fill up From header. If From has already been set, this will fill Sender header. If None, current Sender returned.
  • Envelope(from_=): Sender e-mail or False to explicitly omit. When encrypting without sender, we do not use their key so that we will not be able to decipher again.
  • Envelope(sender=) (see --sender)

  # These statement are identic.
  Envelope(from_ = "identity@example.com")    
  Envelope(sender = "identity@example.com")
  
  # This statement produces both From header and Sender header.
  Envelope(from_ = "identity@example.com", sender="identity2@example.com")        
  

Sending

• send: Send the message to the recipients by e-mail. True (blank in CLI) to send now or False to print out debug information.

  • --send
  • .send(send=True, sign=None, encrypt=None)
    • send: True to send now. False (or 0/false/no in CLI) to print debug information.
    • Returns the object back which converted to bool returns True if the message has been sent successfully.
  • Envelope(send=)

  $ envelope --to "user@example.org" --message "Hello world" --send 0
  ****************************************************************************************************
  Have not been sent from  to user@example.org
  
  Content-Type: text/html; charset="utf-8"
  Content-Transfer-Encoding: 7bit
  MIME-Version: 1.0
  Subject:
  From:
  To: user@example.org
  Date: Mon, 07 Oct 2019 16:13:37 +0200
  Message-ID: <157045761791.29779.5279828659897745855@...>
  
  Hello world
  

• subject: Mail subject. Gets encrypted with GPG, stays visible with S/MIME.

  • --subject
  • .subject(text): If None, current subject returned.
  • Envelope(subject=)

• cc: E-mail or their list. Multiple addresses may be given in a string, delimited by comma (or semicolon). (The same is valid for to, cc, bcc and reply-to.)

  • --cc
  • .cc(email_or_list): If None, current list returned.

    Envelope()
        .cc("person1@example.com")
        .cc("person1@example.com, John <person2@example.com>")
        .cc(["person3@example.com"])
        .cc()  # ["person1@example.com", "John <person2@example.com>", "person3@example.com"] 
    

  • Envelope(cc=)

• bcc: E-mail or their list

  • --bcc
  • .bcc(email_or_list): If None, current list returned.
  • Envelope(bcc=)

• reply-to: E-mail to be replied to or their list. The field is not encrypted.

  • --reply-to
  • .reply_to(email_or_list): If None, current list returned.
  • Envelope(reply_to=)

• date:

  • .date(date) str|False Specify Date header (otherwise Date is added automatically). If False, the Date header will not be added automatically.

• smtp: SMTP server

  • --smtp
  • .smtp(host="localhost", port=25, user=, password=, security=)
  • Envelope(smtp=)
  • Parameters:
    • host may include hostname or any of the following input formats (ex: path to an INI file or a dict)
    • security if not set, automatically set to starttls for port 587 and to tls for port 465
  • Input format may be in the following form:
    • None default localhost server used
    • smtplib.SMTP object
    • list or tuple having host, [port, [username, password, [security]]] parameters
      • ex: envelope --smtp localhost 125 me@example.com will set up host, port and username parameters
    • dict specifying {"host": ..., "port": ...}
      • ex: envelope --smtp '{"host": "localhost"}' will set up host parameter
    • str hostname or path to an INI file (existing file, ending at .ini, with the section [SMTP])

      [SMTP]
      host = example.com
      port = 587            
      

  • Do not fear to pass the smtp in a loop, we make just a single connection to the server. If timed out, we attempt to reconnect once.

  smtp = localhost, 25
  for mail in mails:
      Envelope(...).smtp(smtp).send()
  

• attachments

  • --attachment: Path to the attachment, followed by optional file name to be used and/or mime type. This parameter may be used multiple times.

  envelope --attachment "/tmp/file.txt" "displayed-name.txt" "text/plain" --attachment "/tmp/another-file.txt"
  

  • .attach(attachment=, mimetype=, filename=, path=): Three different usages.
    • .attach(attachment=, mimetype=, filename=): You can put any attainable contents of a single attachment into attachment and optionally add mime type or displayed file name.
    • .attach(mimetype=, filename=, path=): You can specify path and optionally mime type or displayed file name.
    • .attach(attachment=): You can put a list of attachments. The list may contain tuples: contents [,mimetype/filename] [,mimetype/filename].

  Envelope().attach(path="/tmp/file.txt").attach(path="/tmp/another-file.txt")
  

  • Envelope(attachments=): Attachment or their list. Attachment is defined by any attainable contents, optionally in tuple with the file name to be used in the e-mail and/or mime type: contents [,mimetype/filename] [,mimetype/filename]

  Envelope(attachments=[(Path("/tmp/file.txt"), "displayed-name.txt", "text/plain"), Path("/tmp/another-file.txt"])
  

  • mime: Sets contents mime subtype: "auto" (default), "html" or "plain" for plain text. Maintype is always set to "text".
    Set maintype to "text". If a line is longer than 1000 characters, makes the message be transferred safely by bytes (otherwise these non-standard long lines might cause a transferring SMTP server to include line breaks and redundant spaces that might break up ex: DKIM signature).
    In case of Content-Type header put to the message, mime section functionality is skipped.

    • --mime SUBTYPE
    • .mime(subtype="auto", nl2br="auto")
      • nl2br: True will append <br> to every line break in the HTML message. "auto": line breaks are changed only if there is no <br or <p in the HTML message,
    • Envelope(mime=)

  • headers: Any custom headers (these will not be encrypted with GPG nor S/MIME)

    • --header name value (may be used multiple times)
    • .header(name, value=None, replace=False)
      • value If None, returns value of the header or its list if the header was used multiple times. (Note that To, Cc, Bcc and Reply-To headers always return list.)
      • replace If True, any header of the key name are removed first and if val is None, the header is deleted. Otherwise another header of the same name is appended.

      Envelope().header("X-Mailer", "my-app").header("X-Mailer") # "my-app"
      Envelope().header("Generic-Header", "1") \
                .header("Generic-Header", "2") \
                .header("Generic-Header") # ["1", "2"]
      

    • Envelope(headers=[(name, value)])

    Equivalent headers:

    envelope --header X-Mailer my-app
    

    Envelope(headers=[("X-Mailer", "my-app")])
    Envelope().header("X-Mailer", "my-app")
    

Specific headers

These helpers are available via fluent interface.

• .list_unsubscribe(uri=None, one_click=False, web=None, email=None): You can specify either url, email or both.

  • .list_unsubscribe(uri): We try to determine whether this is e-mail and prepend brackets and 'https:'/'mailto:' if needed. Ex: me@example.com?subject=unsubscribe, example.com/unsubscribe, <https://example.com/unsubscribe>
  • .list_unsubscribe(email=): E-mail address. Ex: me@example.com, mailto:me@example.com
  • .list_unsubscribe(web=, one_click=False): Specify URL. Ex: example.com/unsubscribe, http://example.com/unsubscribe. If one_click=True, rfc8058 List-Unsubscribe-Post header is added. This says user can unsubscribe with a single click that is realized by a POST request in order to prevent e-mail scanner to access the unsubscribe page by mistake. A 'https' url must be present.

  # These will produce:
  # List-Unsubscribe: <https://example.com/unsubscribe>
  Envelope().list_unsubscribe("example.com/unsubscribe")
  Envelope().list_unsubscribe(web="example.com/unsubscribe")
  Envelope().list_unsubscribe("<https://example.com/unsubscribe>")
  
  # This will produce:
  # List-Unsubscribe: <https://example.com/unsubscribe>, <mailto:me@example.com?subject=unsubscribe>
  Envelope().list_unsubscribe("example.com/unsubscribe", mail="me@example.com?subject=unsubscribe")
  

• .auto_submitted:

  • .auto_submitted(val="auto-replied"): Direct response to another message by an automatic process.
  • .auto_submitted.auto_generated(): automatic (often periodic) processes (such as UNIX "cron jobs") which are not direct responses to other messages
  • .auto_submitted.no(): message was originated by a human

Envelope().auto_submitted()  # mark message as automatic        
Envelope().auto_submitted.no()  # mark message as human produced

Supportive

• .recipients(): Return set of all recipients – To, Cc, Bcc
  • .recipients(clear=True): All To, Cc and Bcc recipients are removed and the Envelope object is returned.
• .copy(): Return deep copy of the instance to be used independently.

  factory = Envelope().cc("original@example.com").copy
  e1 = factory().to("to-1@example.com")
  e2 = factory().to("to-2@example.com").cc("additional@example.com")  # 

  print(e1.recipients())  # {'to-1@example.com', 'original@example.com'}
  print(e2.recipients())  # {'to-2@example.com', 'original@example.com', 'additional@example.com'}

• Read message and subject by .message() and .subject()

• preview: Returns the string of the message or data as a human-readable text. Ex: whilst we have to use quoted-printable (as seen in str), here the output will be plain text.

  • --preview
  • .preview()

• check: Check all e-mail addresses and SMTP connection and return True/False if succeeded. Tries to find SPF, DKIM and DMARC DNS records depending on the sender's domain and print them out.

  • --check
  • .check(check_mx=True, check_smtp=True)
    • check_mx E-mail addresses can be checked for MX record, not only for their format.
    • check_smtp We try to connect to the SMTP host.

  $ envelope --smtp localhost 25 --sender me@example.com --check 
  SPF found on the domain example.com: v=spf1 -all
  See: dig -t SPF example.com && dig -t TXT example.com
  DKIM found: ['v=DKIM1; g=*; k=rsa; p=...']
  Could not spot DMARC.
  Trying to connect to the SMTP...
  Check succeeded.
  

• .as_message(): Generates an email.message.Message object.

  e = Envelope("hello").as_message()
  print(type(e), e.get_payload())  # <class 'email.message.EmailMessage'> hello\n 
  

• load: Parse any attainable contents (including email.message.Message) like an EML file to build an Envelope object.

  • Limitation: it cannot read an attachment which stays a mere part of the body, it cannot decrypt.
  • Note that if you will send this reconstructed message, you might not probably receive it due to the Message-ID duplication. Delete at least Message-ID header prior to re-sending.
  • (static) .load(message)

    Envelope.load("Subject: testing message").subject()  # "testing message"
    

  • bash
    • allows use blank --subject or --message flags to display the
    • --load FILE

      $ envelope --load email.eml
      Content-Type: text/plain; charset="utf-8"
      Content-Transfer-Encoding: 7bit
      MIME-Version: 1.0
      Subject: testing message
      
      Message body
      
      $ envelope --load email.eml --subject
      testing message          
      

    • (bash) piped in content XXX, envelope executable used with no argument

      $ echo "Subject: testing message" | envelope
      Content-Type: text/plain; charset="utf-8"
      Content-Transfer-Encoding: 7bit
      MIME-Version: 1.0
      Subject: testing message
      
      $ cat email.eml | envelope
      

Default values

In module interface, you may set the defaults when accessing Envelope.default instance.

Envelope.default.subject("Test subject").signature()
Envelope("Hello")  # this message has a default subject and is signed by default when sent

Converting object to str or bool

When successfully signing, encrypting or sending, object is resolvable to True and signed text / produced e-mail could be obtained via str().

o = Envelope("message", sign=True)
str(o)  # signed text
bool(o)  # True

Examples

Signing and encrypting

Sign the message.

Envelope(message="Hello world", sign=True)

Sign the message loaded from a file by standard pathlib library

from pathlib import Path
Envelope(message=Path("/tmp/message.txt"), sign=True)

Sign the message got from a file-stream

with open("/tmp/message.txt") as f:
    Envelope(message=f, sign=True)

Sign and encrypt the message so that's decryptable by keys for me@example.com and remote_person@example.com (that should already be loaded in the keyring).

Envelope(message="Hello world", sign=True
        encrypt=True,
        sender="me@example.com",
        to="remote_person@example.com")

Sign and encrypt the message so that's decryptable by keys for me@example.com and remote_person@example.com (that get's imported to the keyring from the file).

Envelope(message="Hello world", sign=True
        encrypt=Path("/tmp/remote_key.asc"),
        sender="me@example.com",
        to="remote_person@example.com")

Sign the message via different keyring.

Envelope(message="Hello world", sign=True, gnupg="/tmp/my-keyring/")

Sign the message with a key that needs passphrase.

Envelope(message="Hello world", sign=True, passphrase="my-password")

Sign a message without signing by default turned previously on and having a default keyring path. Every Envelope call will honour these defaults.

Envelope.default.signature(True).gnupghome("/tmp/my-keyring")
Envelope(message="Hello world")

Sending

Send an e-mail via module call.

Envelope(message="Hello world", send=True)

Send an e-mail via CLI and default SMTP server localhost on port 25.

envelope --to "user@example.org" --message "Hello world" --send

Send while having specified the SMTP server host, port, username, password.

envelope --to "user@example.org" message "Hello world" --send --smtp localhost 123 username password 

Send while having specified the SMTP server through a dictionary.

envelope --to "user@example.org" --message "Hello world" --send --smtp '{"host": "localhost", "port": "123"}' 

Send while having specified the SMTP server via module call.

Envelope(message="Hello world", to="user@example.org", send=True, smtp={"host":"localhost"}) 

Attachment

You can attach a file in many different ways. Pick the one that suits you the best.

Envelope(attachment=Path("/tmp/file.txt"))  # filename will be 'file.txt'

with open("/tmp/file.txt") as f:
    Envelope(attachment=f)  # filename will be 'file.txt'
    
with open("/tmp/file.txt") as f:
    Envelope(attachment=(f, "filename.txt"))
    
Envelope().attach(path="/tmp/file.txt",filename="filename.txt")

Complex example

Send an encrypted and signed message (GPG) via the default SMTP server, via all three interfaces.

# CLI interface
envelope --message "Hello world" --from "me@example.org" --to "user@example.org" --subject "Test" --sign --encrypt -a /tmp/file.txt -a /tmp/file2 application/gzip zipped-file.zip --send

from pathlib import Path
from envelope import Envelope

# fluent interface
Envelope().message("Hello world").from_("me@example.org").to("user@example.org").subject("Test").signature().encryption().attach(path="/tmp/file.txt").attach(Path("/tmp/file2"), "application/gzip", "zipped-file.zip").send()

# one-liner interface
Envelope("Hello world", "me@example.org", "user@example.org", "Test", sign=True, encrypt=True, attachments=[(Path("/tmp/file.txt"), (Path("/tmp/file2"), "application/gzip", "zipped-file.zip")], send=True)

In the condition me@example.com private key for signing, user@example.com public key for encrypting and open SMTP server on localhost:25 are available, change --send to --send 0 (or .send() to .send(False) or send=True to send=False) to investigate the generated message that may be similar to the following output:

****************************************************************************************************
Have not been sent from me@example.org to user@example.org
Encrypted subject: Test
Encrypted message: b'Hello world'

Subject: Encrypted message
MIME-Version: 1.0
Content-Type: multipart/encrypted; protocol="application/pgp-encrypted";
 boundary="===============8462917939563016793=="
From: me@example.org
To: user@example.org
Date: Tue, 08 Oct 2019 16:16:18 +0200
Message-ID: <157054417817.4405.938581433237601455@promyka>

--===============8462917939563016793==
Content-Type: application/pgp-encrypted

Version: 1
--===============8462917939563016793==
Content-Type: application/octet-stream; name="encrypted.asc"
Content-Description: OpenPGP encrypted message
Content-Disposition: inline; filename="encrypted.asc"

-----BEGIN PGP MESSAGE-----

hQMOAyx1c9zl1h4wEAv+PmtwjQDt+4XCn8YQJ6d7kyrp2R7xzS3PQwOZ7e+HWJjY
(...)
RQ8QtLLEza+rs+1lgcPgdBZEHFpYpgDb0AUvYg9d
=YuqI
-----END PGP MESSAGE-----

--===============8462917939563016793==--

Related affairs

Sending an e-mail does not mean it will be received. Sending it successfully through your local domain does not mean a public mailbox will accept it as well. If you are not trustworthy enough, your e-mail may not even appear at the recipient's spam bin, it can just be discarded without notice.

Configure your SMTP

It is always easier if you have an account on an SMTP server the application is able to send e-mails with. If it is not the case, various SMTP server exist but as a quick and non-secure solution, I've tested bytemark/smtp docker image that allows you to start up a SMTP server by a single line.

docker run --network=host --restart always -d bytemark/smtp   # starts open port 25 on localhost
envelope --message "SMTP test" --from [your e-mail] --to [your e-mail] --smtp localhost 25 --send

Choose ciphering method

Configure your GPG

In order to sign messages, you need a private key. Let's pretend a usecase when your application will run under www-data user and GPG sign messages through the keys located at: /var/www/.gnupg. You have got a SMTP server with an e-mail account the application may use.

ls -l $(tty)  # see current TTY owner
sudo chown www-data $(tty)  # if creating the key for a different user and generation fails, changing temporarily the ownership of the terminal might help (when handling passphrase, the agent opens the controlling terminal rather than using stdin/stdout for security purposes)
GNUPGHOME=/var/www/.gnupg sudo -H -u www-data gpg --full-generate-key  # put application e-mail you are able to send e-mails from
# sudo chown [USER] $(tty)  # you may set back the TTY owner
GNUPGHOME=/var/www/.gnupg sudo -H -u www-data gpg --list-secret-keys  # get key ID
GNUPGHOME=/var/www/.gnupg sudo -H -u www-data gpg --send-keys [key ID]  # now the world is able to pull the key from a global webserver when they receive an e-mail from you
GNUPGHOME=/var/www/.gnupg sudo -H -u www-data gpg --export [APPLICATION_EMAIL] | curl -T - https://keys.openpgp.org  # prints out the link you can verify your key with on `keys.openpgp.org` (ex: used by default by Thunderbird Enigmail; standard --send-keys method will not verify the identity information here, hence your e-mail would not be searchable)
GNUPGHOME=/var/www/.gnupg sudo -H -u www-data envelope --message "Hello world" --subject "GPG signing test" --sign [key ID] --from [application e-mail] --to [your e-mail] --send  # you now receive e-mail and may import the key and set the trust to the key

It takes few hours to a key to propagate. If the key cannot be imported in your e-mail client because not found on the servers, try in the morning again or check the online search form at http://hkps.pool.sks-keyservers.net.
Put your fingerprint on the web or on the business card then so that everybody can check your signature is valid.

Configure your S/MIME

If you are supposed to use S/MIME, you would probably be told where to take your key and certificate from. If planning to try it all by yourself, generate your certificate.pem.

• Either: Do you have private key?

openssl req -key YOUR-KEY.pem -nodes -x509 -days 365 -out certificate.pem  # will generate privkey.pem alongside

• Or: Do not you have private key?

openssl req -newkey rsa:1024 -nodes -x509 -days 365 -out certificate.pem  # will generate privkey.pem alongside

Now, you may sign a message with your key and certificate. (However, the messages will not be trustworthy because no authority signed the certificate.) Give your friend the certificate so that they might verify the message comes from you. Receive a certificate from a friend to encrypt them a message with.

envelope --message "Hello world" --subject "S/MIME signing test" --sign-path [key file] --cert-path [certificate file] --from [application e-mail] --to [your e-mail] --send # you now receive e-mail

DNS validation tools

This is just a short explanation on these anti-spam mechanisms so that you can take basic notion what is going on.

Every time, the receiver should ask the sender's domain these questions over DNS.

SPF

The receiver asks the sender's domain: Do you allow the senders IP/domain to send the e-mail on your behalf?

Check your domain on SPF:

dig -t TXT example.com

DKIM

The receiver asks the sender's domain: Give me the public key so that I may check the hash in the e-mail header that assert the message was composed by your private key. So that the e-mail comes trustworthy from you and nobody modified it on the way.

Check your domain on DKIM:

dig -t TXT [selector]._domainkey.example.com

You can obtain the selector from an e-mail message you received. Check the line DKIM-Signature and the value of the param s.

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=example.com; s=default;

DMARC

What is your policy concerning SPF and DKIM? What abuse address do you have?

Check your domain on DMARC:

dig -t TXT _dmarc.example.com