Possible relay problem sending to external recipients

[Caustic]

I'm running Exchange 2003 sp2 in a larger e-mail organization with multiple Exchange Servers.

When a user sends an e-mail outside the company and specifies both an internal recipient and an external recipient in the "To" field, some external e-mail servers refuse the connection with an error similar to the one below:

There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<cinemail1.na.FKILogistex.Local #5.5.0 smtp;550 Authentication required.

Sometimes that error includes comments about relay attempts but I believe it's being caused by the same phenomenon.

If they send the e-mail with only the external recipient specified and not the internal recipient, the e-mail is delivered fine.

This only happens when sending to some external servers not all. We have appropriate rDNS records setup. E-mail is sent out using public DNS lookups. No smarthost or mail proxy.

It's almost as though the receiving, external e-mail server thinks we're trying to relay the other addresses it doesn't consider local.

Any help is appreciated.

 

[bkolbusz]

We had a similar problem and found out our NAT was setup wrong. The Exchange server was going out as the Public IP on the router not the NAT'd IP. Try going to

http://www.ipchicken.com

and verify you are going out how you think you are going out.