SMTP communication problem with the recipient's email server

[Nyn11Tech]

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 difficlut 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.

[DigitalPrimate]

I am having a similar issue with one recipient. We are also using Exchange 2003 SP2. It appears the recipient has a working SMTP server on the server hosting their website however their actual mail server resides at a different IP. NSLOOKUP of their MX record returns the proper mail server IP, however NSLOOKUP of their root domain record (domainname.com) returns the IP address of their website server.

In our case, for some reason, Exchange appears to try connecting to the IP address of their website about 30% of the time instead of the IP associated with the MX record for the domain. The SMTP running on the webserver responds, however since it is not configured to support the domain in question, emails get rejected. The other 70% of the time, Exchange connects to the proper IP address and emails get delivered.

Temporarily I am going to relay emails for this domain off my ISP's SMTP using an SMTP connector to see if this resolves the issue for me, however either something is fishy in their DNS configuration or Exchange is acting stupid when resolving the domain's mail server IP.

Still researching on how exactly Exchange handles that DNS lookup process...