Question : 550 5.1.1 Recipient address rejected:

I have a bit of a puzzle on my hands...  I can receive email from most of the free world, but I have several customers who receive the following error message when sending me an email:
550 5.1.1 : Recipient address rejected: efg.com

We have not made any changes to our email setup in the last three weeks.   We moved from an external ISP (POP3) to an internal Exchange 2003 server.  

Any suggestions on which way to run with this one would be greatly appreciated.

Answer : 550 5.1.1 Recipient address rejected:

After pounding my head on the table for a few hours I found the cause...

Someone had change the mx record.  It was pointing to a server where the email address did not exist.  Thus rejecting the recipient.