My wife receives this message when replying to various emails. Doesn't happen all the time. No pattern (yet) that I can see other than she is replying. There are not multiple addresses, only one.

From: System Administrator
Sent: Friday, January 27, 2006 8:28 AM
To: 'SHEILA'
Subject: Undeliverable: Feb 1st

Your message did not reach some or all of the intended recipients.

Subject: RE: Feb 1st

Sent: 1/27/2006 8:28 AM

The following recipient(s) could not be reached:

'SHEILA' on 1/27/2006 8:28 AM

550 #5.1.0 Address rejected.

If she uses the "resend" option for this message it goes through with no problem. Any clues?