[GSoC21 Application Task] Email server app (/apps/email_server) with nullmailer

This is an application task for Google Summer of Code 2021. It has no web UI but provides a set of actions to configure and test the Nullmailer MTA. Installation is required. Its landing page is at http://freedombox.local/plinth/apps/email_server

The action implements three functionalities: --reconfigure, --diagnose and --send-test-email-to [addr]. After installing the app, wait for Plinth to download a few dependencies. Then, run the --reconfigure option or follow this wiki page to set up your Nullmailer MTA.

$ sudo /freedombox/actions/email_server --reconfigure
-- Information about this server --
Server FQDN: test.localhost
Admin email address: RecipientEmailExample@debian.org
-- Information about your email account --
SMTP server: smtp.debian.org
SSL/TLS port [465]:
Email address [RecipientEmailExample@debian.org]:
Password or access token:
Config updated

Test your configuration by sending an email.

sudo /freedombox/actions/email_server --diagnose
/freedombox/actions/email_server --send-test-email-to RecipientEmailExample@debian.org

Replace RecipientEmailExample@debian.org with a working email address. You may or may not need root permission to send the email.

Exit codes

The following exit codes are returned by email_server to indicate failures.

• 1 (EXIT_FAIL) Generic failure
• 10 (EXIT_SYNTAX) Syntax error, invalid input

Python API

import plinth.modules.email_server.actions as esa
esa.send_test_email_to('RecipientEmailExample@debian.org')

send_test_email_to(to_address) - Send a test email to the specified address. This function requires working Nullmailer configuration. The subject and body of test emails cannot be changed.

• parameter str to_address: email address for test notification
• returns: exit code of mail (not the action). Non-zero value indicates failure
• raises ValueError: if it encounters an invalid email address
• raises Exception: for other errors (such as file access problems)

Troubleshooting

Some providers reject emails that contain unrecognized sender addresses. Put a valid sender address/return path in /etc/nullmailer/adminaddr. Your test email's From header will be set to your adminaddr.

Still having trouble? Run sudo service nullmailer status to check delivery logs.

Limitations

• Only plaintext auth is supported
• TLS must be used
• Cannot have whitespace, quotation marks, backward slashes, control characters or non-ASCII characters in the password
• Similar restrictions apply to the email address

Miscellany

• In /etc/nullmailer/remotes, the syntax for using TLS (instead of STARTTLS opportunistic encryption) is like:

  smtp.example.com smtp --port=465 --auth-login --ssl --user=you@example.com --pass=Password

• GMail logs the SMTP client's IP address in the Received header for anti-phishing purposes. If you are a GMail user and do not want that, then use a VPN or only send test emails to yourself.

References

• Debian Wiki "nullmailer"