This subthread moved from Mailman3 Users list.
ASIDE: This got caught for several days in a mismatch between Exim and greylisting. If you're an Exim user, recent versions are pretty aggressive about using the PIPELINING and CHUNKING extensions to SMTP, and one or both of those seems to be a problem.
The symptom was repeated 4xx rejections of each message, and communicating with Postmaster revealed that the greylist software complained about possible PIPELINING after a BDAT command (which is part of the CHUNKING protocol). If you're running a low-traffic MTA you might want to disable these extensions. That's general advice if you use any MTA that implements them, not limited to Exim.
Mark Sapiro writes in that thread:
self._charset is the character set of the lists preferred_language. The footer can't contain characters not in the character set of the list's preferred_language. For English (en) this is us-ascii. For German (de) it is iso-8859-1. These are artifacts from Mailman 2.1
ISO-8859-15 (has EURO SIGN substituted for generic CURRENCY SYMBOL and other minor changes) should probably be considered for substitution for ISO-8859-1 in Mailman 2.
What do Western Europeans think? If someone's concerned, I can look up the other differences from ISO-8859-1. I believe they're quite minor populations of users compared to the Euro sign users, but backward compatibility, etc.
You can change this for English by putting
[language.en] charset: utf-8
in your mailman.cfg. This works because us-ascii is a proper subset of utf-8.
For other languages, a change like this might require recoding of templates and/or a message catalog, but since we don't currently ship any non-English templates or message catalogs, that is moot at the moment.
We should simply declare that template files in Mailman 3 MUST be encoded in UTF-8 (or US-ASCII). I'll file an RFE shortly, including a suggestion that we provide a detection (report only!) and conversion utility.