From my own domain/LAN (Exchange 2003)...
When trying to send an outgoing email to one certain domain it fails and we receive this error message back:
Your message could not be sent.
A transcript of the attempts to send the message follows.
The number of attempts made: 1
Addressed To: firstname.lastname@example.org
Wed, 11 Feb 2009 09:17:09 -0500
Failed to send to identified host,
email@example.com: [outgoingdomain.com.s5a1.psmtp.com], 554 No relaying allowed - psmtp
--- Message non-deliverable.
Also, I'm not blacklisted. I checked that on mxtoolbox.com.