Replying to myself,
Jun 8 07:10:03 ip-10-160-125-11 postfix/smtp[4735]: > example.com[72.34.48.xxx]:25: DATA Jun 8 07:10:03 ip-10-160-125-11 postfix/smtp[4735]: < example.com[72.34.48.xxx]:25: 250 OK
I believe that this is the problem, the correct response from the "busted server" should have been 354, but for some reason it's dropped a 250.
Am I right?I can confirm that if the PIPELINING feature is ignored by postfix, then everything is fine.
