Default auto reply trigger failure

Ah thanks for the further input, that helped!
We’re not sending automatic trigger based notifications to that mail, because it’s from a mailing list:

This is done by postmaster filters and shouldn’t be changed to ensure nothing “unexpected” happens.
I’m not sure why you’d want to reply to a mailing list, it should work if it’s not coming from a bulk message or list.

The two parts that are responsible in Zammad are the following ones:

and to be more precise to know which headers are involved: