I have come across some sites who reject mail from domains which list one or more ipv6-only MX servers. The rejection message is similar to the following:- <recipient@example.com>: host broken-server.example.com[10.20.30.40] said: 554 5.7.1 Sorry; <sender@example.com> has an invalid MX record: MX Record: ipv6-mx.example.com. does not resolve. (in reply to MAIL FROM command) In this example, ipv6-mx.example.com. has a valid AAAA record and no A record. If you run a mail server you might want to check if you are affected as you may be rejecting messages without realizing it. I have found that this also affects messages to postmaster@ at the affected domains, which in my interpretation violates rfc 2821.