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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")
    .attach(file_contents, filename="attached-file.txt")
    .smtp("localhost", 587, "user", "pass", "starttls")


  • 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+
    • Or just download the project and launch ./
  • 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


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


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

envelope --message "Hello world" \
               --output "/tmp/output_file" \
               --sender "" \
               --to "" \
               --encrypt-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.

import envelope
envelope(message="Hello world",

Module: fluent interface

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

import envelope
envelope().message("Hello world")\

Note: if autocompletion does not work, use from envelope import envelope instead of import envelope.
(For example, Jupyter can autocomplete with import envelope but PyCharm cannot because it does not serves itself with a running kernel.)


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
  • envelope(param=) is a one-liner argument
  • .param(value) denotes a positional argument
  • .param(value=) denotes a keyword argument

Any fetchable 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.

    • --message: String
    • --input: (CLI only) Path to the message file. (Alternative to --message parameter.)
    • envelope(message=): Any fetchable contents
    • .message(text): String or stream.
    • .message(path=None): Path to the file.

    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
    • envelope(output=)
    • .output(output_file)

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]
    • envelope(gpg=True)
    • .gpg(gnugp_home=True)
  • .smime: Prefer S/MIME over GPG
    • --smime
    • envelope(smime=True)
    • .smime()


  • sign: Sign the message.
    • --sign:
      • "auto" for turning on signing if there is a key matching to the "from" header
      • GPG: Blank for user default key or key ID/fingerprint.
      • S/MIME: Any fetchable contents with key.
    • --sign-path: S/MIME: 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.)
    • envelope(sign=):
      • GPG: True for user default key or key ID/fingerprint.
      • S/MIME: Key contents.
    • 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 fetchable contents.
    • .sign(key=True, passphrase=, attach_key=False, cert=None, key_path=None): Sign now (and you may specify the parameters)
    • .signature(key=True, passphrase=, attach_key=False, cert=None, key_path=None): Sign later (when launched with .sign(), .encrypt() or .send() functions


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.
    • --encrypt: Key string or blank or 1/true/yes if the key should be in the ring from before. Put 0/false/no to disable encrypt-path.
    • --encrypt-path (CLI only): Recipient public key stored in a file path. (Alternative to --encrypt.)
    • envelope(encrypt=): Any fetchable contents
    • .encrypt(key=True, sign=, key_path=): With sign, you may specify boolean or default signing key ID/fingerprint for GPG or Any fetchable contents with S/MIME key + signing certificate. If import needed, put your encrypting GPG key contents or S/MIME certificate to key or path to the key/certificate contents file in key_path.
    • .encryption(key=True, key_path=): Encrypt later (when launched with .sign(), .encrypt() or .send() functions. If needed, in the parameters specify Any fetchable contents with GPG encryption key or S/MIME encryption certificate.
  • to: E-mail or list. When encrypting, we use keys of these identities.
    • --to: One or more e-mail addresses.
    • envelope(to=): E-mail or their list.
    • .to(email_or_list):
      envelope --to --message "hello" 
  • sender: E-mail – needed to choose our key if encrypting.
    • --sender E-mail
    • --no-sender Declare we want to encrypt and never decrypt back.
    • --from Alias for --sender
    • envelope(sender=): 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.
    • .sender(email): E-mail or False.
    • .from_(email): an alias for .sender


  • 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
    • envelope(send=)
    • .send(send=True, sign=None, encrypt=None)
      • send: True to send now. False (or 0/false/no in CLI) to print debug information.
    $ envelope --to "" --message "Hello world" --send 0
    Have not been sent from  to
    Content-Type: text/html; charset="utf-8"
    Content-Transfer-Encoding: 7bit
    MIME-Version: 1.0
    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
    • envelope(subject=)
    • .subject(text)
  • cc: E-mail or their list

    • --cc
    • envelope(cc=)
    • .cc(email_or_list)
  • bcc: E-mail or their list

    • --bcc
    • envelope(bcc=)
    • .bcc(email_or_list)
  • reply-to: E-mail to be replied to. The field is not encrypted.

    • --reply-to
    • envelope(reply_to=)
    • .reply_to(email)
  • smtp: SMTP server

    • --smtp
    • envelope(smtp=)
    • .smtp(host="localhost", port=25, user=, password=, security=)
    • 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 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])
        host =
        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:
  • 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"
    • gpggp(attachments=): Attachment or their list. Attachment is defined by any fetchable contents, optionally in tuple with the file name to be used in the e-mail and/or mime type: content [,name] [,mimetype]
    envelope(attachments=[(Path("/tmp/file.txt"), "displayed-name.txt", "text/plain"), Path("/tmp/another-file.txt"])
    • .attach(attachment_or_list=, path=, mimetype=, filename=): Three different usages.
      • .attach(attachment_or_list=, mimetype=, filename=): You can put Any fetchable contents in attachment_or_list and optionally mimetype or displayed filename.
      • .attach(path=, mimetype=, filename=): You can specify path and optionally mimetype or displayed filename.
      • .attach(attachment_or_list=): You can put a list of attachments.
    • mime: Set contents mime subtype: "auto" (default), "html" or "plain" for plain text. Ignored if Content-Type header put to the message.

      • --mime SUBTYPE
      • envelope(mime=)
      • .mime(subtype="auto", nl2br="auto")
        • nl2br: True: envelope 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,
    • headers: Any custom headers (these will not be encrypted with GPG nor S/MIME)

      • --header name value (may be used multiple times)
      • envelope(headers=[(name, value)])
      • .header(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:,, <>
    • .list_unsubscribe(email=): E-mail address. Ex:,
    • .list_unsubscribe(web=, one_click=False): Specify URL. Ex:, 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: <>
    # This will produce:
    # List-Unsubscribe: <>, <>
    envelope().list_unsubscribe("", mail="")
  • .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
    • message was originated by a human
envelope().auto_submitted()  # mark message as automatic        
envelope()  # mark message as human produced


  • .recipients(): Return set of all recipients – To, Cc, Bcc

    • .recipients(clear=True): All To, Cc and Bcc recipients are removed and the object is returned.
  • Read message and subject by .message() and .subject()

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

  • check: Check SMTP connection and returns 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()
    $ envelope --smtp localhost 25 --sender --check 
    SPF found on the domain v=spf1 -all
    See: dig -t SPF && dig -t TXT
    DKIM found: ['v=DKIM1; g=*; k=rsa; p=...']
    Could not spot DMARC.
    Trying to connect to the SMTP...
    Check succeeded.
  • static .load(message) (experimental): Parse any fetchable contents like an EML file to build an Envelope object.

    • Still considered experimental: 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.
    envelope.load("Subject: testing message").subject()  # "testing message"

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


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 and (that should already be loaded in the keyring).

envelope(message="Hello world", sign=True

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

envelope(message="Hello world", sign=True

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(message="Hello world")


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 "" --message "Hello world" --send

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

envelope --to "" message "Hello world" --send --smtp localhost 123 username password 

Send while having specified the SMTP server through a dictionary.

envelope --to "" --message "Hello world" --send --smtp '{"host": "localhost", "port": "123"}' 

Send while having specified the SMTP server via module call.

envelope(message="Hello world", to="", send=True, smtp={"host":"localhost"}) 


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"))

Complex example

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

# CLI interface
envelope --message "Hello world" --to "" --sender "" --subject "Test" --sign --encrypt -a /tmp/file.txt -a /tmp/file2 application/gzip --send
# one-liner interface
from pathlib import Path
envelope().message("Hello world").to("").sender("").subject("Test").signature().encryption().attach(path="/tmp/file.txt").attach(Path("/tmp/file2"), "application/gzip", "").send()

# fluent interface
envelope(message="Hello world", to="", sender="", subject="Test", sign=True, encrypt=True, attachments=[(Path("/tmp/file.txt"), (Path("/tmp/file2"), "application/gzip", "")], send=True)

In the condition private key for signing, 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 to
Encrypted subject: Test
Encrypted message: b'Hello world'

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

Content-Type: application/pgp-encrypted

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




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.

GNUPGHOME=/var/www/.gnupg sudo -H -u www-data gpg --full-generate-key  # put application e-mail your are able to send the e-mail from
# if the generation fails now because you are on a remote terminal, you may want to change temporarily the ownership of the terminal by the following command: 
# sudo chown www-data $(tty)  # put it back afterwards
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 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
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.


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


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]

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;; s=default;


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

Check your domain on DMARC:

dig -t TXT
You can’t perform that action at this time.