messagelabs.com contact - SMTP-side domaincheck checks IPv4 only, rejects domains with first MX on IPv6

As the subject states, MAILER-DAEMON@messagelabs.com: <....@ford.com>: Connected to 136.1.7.8 but sender was rejected. Remote host said: 501 Sender domain must exist As it obviously checks only the first MX record if there are A records, and if there are none it rejects it. This while there are AAAA records on the first MX, and even A records on the remaining MXs. Thus a proper fix would already be to check the other MXs and of course to check for AAAA too ;) And that affects all customers at messagelabs, thus if somebody can pass that along to them to fix it, that would be great ;) Oh and of course the check is also there for postmaster@ thus no way to tell them through that route. Greets, Jeroen