Note: This is my re-post from Stackoverflow.

I've been messing with a test environment for security purposes where a DMZ RHEL5 sendmail server is used as a relay for an Exchange 2007 server. Exchange is working in the environment, I have Vista and XP VMs using Outlook on the Domain to send e-mail to each other. I've been trying to simulate an external internet VM sending an e-mail to the DMZ sendmail relay, which forwards to the Exchange server.

Before everyone thinks this is too big a problem/question, I've followed the sendmail/Exchange guides and all I want to know is how I can determine why a relayed message/e-mail in Exchange is "Undelivered".

Basically I send a SMTP message to the sendmail server, which relayed to my Exchange. The /var/log/maillog shows the e-mail being relayed to Exchange.

Nov 17 13:41:22 externalmailserver sendmail[9017]: pAHIfMuW009017: from=<[email protected]>, size=1233, class=0, nrcpts=1, msgid=<[email protected]>, proto=ESMTP, daemon=MTA, relay=[10.50.50.1]
Nov 17 13:42:17 externalmailserver sendmail[9050]: pAHIfMuW009017: to=<[email protected]>, delay=00:00:55, xdelay=00:00:36, mailer=relay, pri=121233, relay=mailserver.xyz.local. [192.168.1.20], dsn=2.0.0, stat=Sent (<[email protected]> Queued mail for delivery)

This is good, but the To never receives the e-mail from Exchange. So I started poking around Exchange. In the "Message Tracking" Troubleshooting Assistant I queried the processed messages and found this: (I had to copy and paste the cells... sorry for the format)

2011/11/17 RECEIVE SMTP <[email protected]> "Undelivered Mail Returned to Sender" [email protected] [email protected] 192.168.100.10 MAILSERVER\DMZ Relay [email protected]

I just want to know if anyone has any suggestions on why the DMZ Relay Connector I setup isn't relaying and is instead returning the forwarded e-mail to sender as Undelivered?

My Exchange Relay Receive Connector is pretty simple. The Exchange server's FQDN is set as the HELO response, all available IP addresses can receive relayed e-mail, and the IP address of my sendmail server is specifically set as a remote server.