It is possible to add addresses to *_these_nonmembers from the admindb interface that cause later problems.

[Mark Sapiro]

The admindb summary page has a checkbox and radio buttons to add the sender address of a held non-member post to one of the *_these_nonmembers filters. Sometimes the sender address will fail Mailman's syntactic address validation, e.g. because it doesn't have a qualified domain name. This addition is accepted and causes no real problem until the next time an admin tries to modify the filter via the admin web UI, at which point all the addresses are checked and the change rejected.