Or you should avoid looking up AAAA at all, this saves time and ressources
What transports are you using? Check your log file, there should be a notice about the transport that was used to deliver the message, T=...
And then inspect that transport exim -bP transports remote_smtp ... or whatever
Rubin: I'd say, after reading the above mentioned document, it's really a host error. (Of course, in your situation it's bound to the message), but aren't they violating some standards, dropping the SMTP connection during the DATA phase?