MegaBrutal via Mailman-users writes:
What I found out is that the second address must match the pattern "Some Name" <a.b0@example.com>. The e-mail is properly formatted when it gets into Mailman, but when it is sent out to list members, it gets malformed to Some@serverhostname.example.com.
This looks more like the kind of thing that an MTA sometimes does, rather than Mailman. Mailman does not modify addresses ever as far as I know, although in some cases it will substitute a different, fixed address for the original one in the message. (The three cases I know of are rewriting From to contain the list address for anonymous lists and DMARC mitigation, and it can be configured to substitute a fixed address in Reply-To.) Mailman doesn't do the kind of editing you describe, as far as I'm aware.
Is example.com your domain? If you can look at the MTA configuration for that domain, check for address rewriting in the configuration.
The easiest way to test it is to install the Mailhog application on the same host as Mailman, then configure Mailhog to send to the same SMTP gateway as Mailman, and configure Mailman to send to localhost and the Mailhog port. Mailhog will then capture Mailman's outgoing mail, you can check it in Mailhog's queue directory. Then you can release any non-test emails from Mailhog. If the messages captured by Mailhog show the problem, then it's probably Mailman.
You can also get the same effect by just shutting down the MTA, and looking at the message in Mailman's 'out' queue (or perhaps the MTA's 'in' queue for some MTAs). But that would affect all incoming and outgoing mail, not just Mailman.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan