We receive these mail rejects periodically from only one recipient. All other mail works fine and has been fine for 4 years. When we receive this reject, subsequent mail to this recipient works for one to several days, then we get a reject again. It may actually reset daily - we are a 24-hour operation so it is difficULt to tell. We will get these rejects for mail sent to any user in the recipient's domain.
The reject message we get is:
The following recipient(s) cannot be reached:
Sample User on 11/3/2008 9:34 AM
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<email.int.ourdomain.org #5.5.0 smtp;503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.>
We are running MS Server 2003 running Exchange Server 2003 SP2.
The recipient is using a Barracuda device and says when this happens he does not see the mail even hitting his filter.
I can successfully perform an NSLookup internally and it resolves to the recipient correctly.
Any ideas would be GREATLY appreciated.