Mail fun and Sender address Rejected

So we had some issues with some one else’s mail server not accepting mail from us. the error was:

 Sender address rejected: Sender domain does not appear to have a SMTP server running on any MX points (in reply to RCPT TO command))

Apparently it did not like mail coming from a non existent account on a domain that did not accept mail! (who would have thought that spam checkers would be so thorough! *hangs head in shame*)

The fix was pretty simple. I setup some postfix smtp generics mapping

Pro Tip: Make sure you use postmap on the generics file!

postmap /etc/postfix/generic

not doing this makes mail not send and clients unhappy!