Jana Berger writes:
It was (until now) only reported from this domain, but Nemesis is used by GMX, one of the largest mail providers in Germany. I am reluctant to pin this issue on their end for now.
If it's only reported from one domain, and has never been reported for Mailman 3 before, I'm reluctant to pin this issue on Mailman.
I am using the "docker-mailman" image from https://github.com/maxking/docker-mailman
It is using Python 3.11 with Alpine 3.18 as its base.
OK, that's helpful, we have a good baseline.
I am using Postfix as the main MTA which is set to "smtputf8_enable = yes" together with "smtputf8_autodetect_classes = sendmail, verify". Is this an issue?
Given that the problem is pure ASCII text that as far as we know leaves Mailman labeled as US-ASCII/7bit, but later is being labeled as UTF-8 and base64-encoded -- yes, that's evidence.
Whether it's related to the issue or not, I don't know, I have no experience with Postfix's implementation of SMTPUTF8. If I understand the Postfix documentation[1] correctly, it *should* be irrelevant to Mailman since your configuration doesn't include smptd, Mailman sends outgoing mail by smtpd, and Mailman doesn't user SMTPUTF8. It appears that Postfix is conservative about SMTPUTF8 and does not use it for outgoing message unless required by the message at receipt.
The only thing I can think of at the moment that might move the discussion forward is if Mark knows how to run his test in your Docker environment to see if it also produces Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit
Footnotes: [1] https://www.postfix.org/postconf.5.html