Emails mixmaster sends has no indications that email is auto-generated and shouldn't be replied to by other programs. For example the following message:
This message is being sent to you automatically in response to an email
that you sent to <...>.
Most likely, you tried to reply to an email that has been sent through
this service. If you did not send an email to <...>,
please ignore this message.
This result in mail loops between programs.
See the following RFC that gives a few recommendations and describes Auto-Submitted header field.