This directory contains a docker compose file that will spawn kumod (the MTA process) in a sink configuration. The SMTP service will pretend to accept most mail and then simply discard it. For the messages it doesn't accept, it will return a failure response based on the contents of the policy/responses.toml file.

That file encodes the chances of returning a bounce or a transient failure, with that chance being configurable based on the recipient domain. In addition, certain recipient domains include a sample of realistic error responses that have been observed to be produced by that domain in real production traffic.

In addition to that probabilistic bounce behavior, you can explicitly choose a class of response by varying the user portion of the recipient address:

if the user includes tempfail a 400 tempfail requested response will be generated
if the user includes permfail a 500 permfail requested response will be generated
If the user starts with 450- then response will be 450 you said USER.
If the user starts with 250- then message will be accepted and discarded, ignoring the probabilistic bounce settings.

Use this to start the daemon and view the logs:

$ docker compose up

The service will spawn and bind to port 2525 on the host system.

You can then send mail on port 2525, using swaks:

$ docker run --rm -it --net host nicolaka/netshoot \
    swaks -f user@example.com -t user@example.com \
    --server $HOSTNAME:2525