Comment 5 for bug 1287604

The major issue with your patch is not all valid email domains have MX records. A valid email domain must have either an A record, an AAAA record or an MX record pointing to a host name with an A or AAAA record. E.g., a domain with an MX pointing to a host name with only a CNAME or MX is not a valid email domain, but a domain with only an A or AAAA record is a valid email domain. A domain which has a CNAME is a valid email domain as long as the CNAME directly or indirectly points to a host with an A, AAAA or MX as above. See RFC 5321, Sec 5.1.

Another issue is Mailman tries not to use bare except: clauses to catch exceptions. We prefer to catch only those explicit exceptions which might occur. I.e., in this case, explicitly only those exceptions that might be returned by dns.resolver.query.