I've looked all over EE for the answer to this one with no luck. We are not able to send e-mail to one specific domain which I will call domain.com. When sending an e-mail, an Undeliverable message comes back immediately:
Your message did not reach some or all of the intended recipients.
Subject: Test Message from United Seminary
Sent: 11/27/2006 9:29 AM
The following recipient(s) could not be reached:
email@example.com on 11/27/2006 9:29 AM
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail.unitedseminary.edu #5.5.0 smtp;550 - Requested action not taken: relaying denied>
This message is immediate and generated by our Exchange server. People from this domain are able to send e-mail into us. I've checked and neither of us are blacklisted.