Need help with Error: 550 relaying denied

We have a user, Mel@clarkstate.edu, that sends emails to a group of people with different domains. Once in a while, Mel receives a bounce back with a 550 relaying denied when sending to Eli@keglerbrown.com. Yet, the email is delivered to the other addresses (different domains) without a problem. Sometimes email will be delivered to Eli with no bounce back. In fact, if Mel gets the 550 error then resends the same email, it will go through.

Mel is using the same computer to send all email, uses the GroupWise 7.x client.

Our email goes from the GroupWise email server to our Barracuda Spam Firewall. Barracuda relays all inbound and outbound mail.

Can someone help me figure out what the problem is?

I have included excerpts from the GWIA log file below, where two email messages could not be sent and a 550 relaying denied message was generated.

What does the following line represent from the GWIA log file?

Response: 220 mta145.mail.ac4.yahoo.com ESMTP YSmtp service ready

GWIA LOG FILE EXCERPTS:

7/17/09
10:12:26 01A MSG 53860 Analyzing result file: RHMAIL2/VOL_MAIL:\GWAPPS\GWDOMDIR\WPGATE\GWIA\result\ra604 e6e.694
10:12:26 01A MSG 53860 Command: keglerbrown.com
10:12:26 01A MSG 53860 Response: 220 mta145.mail.ac4.yahoo.com ESMTP YSmtp service ready
10:12:26 01A MSG 53860 Command: EHLO csccmail.clarkstate.edu
10:12:26 01A MSG 53860 Response: 250 ok
10:12:26 01A MSG 53860 Command: MAIL FROM:<mel@clarkstate.edu>
10:12:26 01A MSG 53860 Response: 250 sender <mel@clarkstate.edu> ok
10:12:26 01A MSG 53860 Detected error on SMTP command
10:12:26 01A MSG 53860 Command: RCPT TO:<eli@keglerbrown.com>
10:12:26 01A MSG 53860 Response: 550 relaying denied for <eli@keglerbrown.com>
10:12:26 01A MSG 53860 Command: DATA
10:12:26 01A MSG 53860 Response: 221 mta145.mail.ac4.yahoo.com
10:12:26 01A Building undeliverable message
10:12:26 01A MSG 53860 Converting message to MIME: RHMAIL2/VOL_MAIL:\GWAPPS\GWDOMDIR\WPGATE\GWIA\send\xa604e8 a.698
10:12:26 01A MSG 53860 Queuing message to daemon: RHMAIL2/VOL_MAIL:\GWAPPS\GWDOMDIR\WPGATE\GWIA\send\sa604e8 a.698

7/28/09
09:25:44 01A MSG 77178 Analyzing result file: RHMAIL2/VOL_MAIL:\GWAPPS\GWDOMDIR\WPGATE\GWIA\result\ra6ec 3fa.899
09:25:44 01A MSG 77178 Command: keglerbrown.com
09:25:44 01A MSG 77178 Response: 220 mta353.mail.mud.yahoo.com ESMTP YSmtp service ready
09:25:44 01A MSG 77178 Command: EHLO csccmail.clarkstate.edu
09:25:44 01A MSG 77178 Response: 250 ok
09:25:44 01A MSG 77178 Command: MAIL FROM:<Mel@clarkstate.edu>
09:25:44 01A MSG 77178 Response: 250 sender <mel@clarkstate.edu> ok
09:25:44 01A MSG 77178 Detected error on SMTP command
09:25:44 01A MSG 77178 Command: RCPT TO:<eli@keglerbrown.com>
09:25:44 01A MSG 77178 Response: 550 relaying denied for <eli@keglerbrown.com>
09:25:44 01A MSG 77178 Command: DATA
09:25:44 01A MSG 77178 Response: 221 mta353.mail.mud.yahoo.com
09:25:44 01A Building undeliverable message
09:25:44 01A MSG 77178 Converting message to MIME: RHMAIL2/VOL_MAIL:\GWAPPS\GWDOMDIR\WPGATE\GWIA\send\xa6ec41 8.904
09:25:44 01A MSG 77178 Queuing message to daemon: RHMAIL2/VOL_MAIL:\GWAPPS\GWDOMDIR\WPGATE\GWIA\send\sa6ec41 8.904