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Fake SMTP server

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nullsmtpd is a fake SMTP server (written in python) which can be used for development. The server catches incoming mail, writes them to a file, and doesn’t actually send it out to the intended recipients. All emails are saved to a directory (defaults to /var/log/nullsmtpd, but can be changed via flag) where each email address sent to gets its own folder and then emails are stored in that folder named {sender}.{time}.msg. This is useful for debugging applications where you don’t want a GUI (as you’re running a headless VM say), but still actually need to see the messages that were sent in the system, without having to worry about things getting bounced or whatever.

Generally, you’d probably want to install this instead of something like sendmail or some other package that would handle sending mail from your server.


nullsmtpd relies on Python 3.5+ and aiosmtpd. Recommended to also have pip for installation.


Recommended Installation is through PyPi:

pip install nullsmtpd

Or if you’ve cloned the repository:

python3 install


usage: nullsmtpd [-h] [--no-fork] [-H HOST] [-P PORT] [--mail-dir MAIL_DIR]

optional arguments:
  -h, --help            show this help message and exit
  --no-fork             Don't fork and run nullsmtpd as a daemon. Additionally,
                        this willprint all log messages to stdout/stderr and
                        all emails to stdout.
  -H HOST, --host HOST  Host to listen on
  -P PORT, --port PORT  Port to listen on
  --mail-dir MAIL_DIR   Location to write logs and emails (defaults to
  -v, --version         show program's version number and exit

By default, you will need to use sudo as the server wants to bind itself to localhost and port 22. However, if you use a different port, then it will not require using sudo. However, note, that if the current running user of the script does not have access to create a directory/file within /var/log or /var/log/nullsmtpd, then no emails will be written to file, but just through the console.

If the mail directory is writeable to, then you should have the following directory structure:


where nullsmtpd.log a log file for the system, and then each folder represents a person’s “inbox” and inside are messages from someone at a given timestamp (seconds from epoch).

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