I have a user thats every now and then gets this after he sends an email:
From: System Administrator
Sent: 03 November 2011 15:21
Subject: Undeliverable: Forwar me the email
Your message did not reach some or all of the intended recipients.
Subject: FW: Forwar me the email
Sent: 03/11/2011 15:21
The following recipient(s) could not be reached:
() on 03/11/2011 15:21
None of your e-mail accounts could send to this recipient.
But the email is always delivered to the recipient.