Dear Mark,
thank you very much for your hints. Every issue has now been sorted out and settled.
The problem was mainly caused by the long MX record TTL: I fixed the issue on the new(!) server successfully, but the mail was sent to the _unfixed_ old server due to the very high TTL time of the MX record!
Thus I was testing by mailing to the unfixed server for some hours, stupid, but true.
Happy end: I backported your suggested database fix to the old server and found that the utf8mb4 option was missing there - other than I said. I was really puzzled due to the MX time-to-lease issue and I *deeply* apologize for bringing some trouble here to the mailing list.
Hint for people who might have the same problem:
- This is good to have in /etc/mailman3/settings.py:
'OPTIONS': { 'charset': 'utf8mb4', 'init_command': "SET sql_mode='STRICT_TRANS_TABLES'", }
and
- The database fix is this one, please take note that the *longtext* verb is needed to make the command working on Server version: 10.11.14-MariaDB-0+deb12u2 Debian 12
ALTER TABLE hyperkitty_email DEFAULT CHARACTER SET utf8mb4, MODIFY subject varchar(512) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci, MODIFY content longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;