Further testing of Mailman 3 uncovered a bug in how Skara represents email lists in an email list server. In the configuration it's defined as an email address, but then we cut out the domain part and only store the local part of the email as a String. Later when processing emails, the domain of the mailing list server is implicitly added to form an email address when needed. This has worked so far, because the domain of every mailing list has been the same as the domain of the archive server (openjdk.org), but during testing, we have a mix of mailing list names and test servers where this isn't matching, so we need to handle it correctly.

The fix is to change the type and representation of mailing lists to be an actual EmailAddress. It resolves all the problems from what I can tell. It does create a rather big patch however.