we have approx. 20K mails per day and most of the time all is ok, but we have one domain which makes problems:
Their mailserver responds:
Remote-MTA: dns; wppim001.aexp.com
Diagnostic-Code: smtp; 530 Must issue a STARTTLS command first
So, I have no idea why it would fail for this mail connection. Any ideas on what to check? The problem is, as this is american express, we can not just send test-mails either, so it's a bit dificult for me to know exactly what's going on, any ideas?