Check validity of emails in some way

[17:14:52] paulproteus, phildini: one annoying thing about the current debexpo is that it doesn't do VERP/bounce-detection so we sysadmins get a lot of bounces from folks who signed up with a bogus email address [17:15:10] if you could steal the code for that from mailman3 it would be awesome

[ originally reported by Philip James at https://github.com/debexpo/debexpo/issues/9 ]