550 relay error when sending multiple e-mails

[jason@xxxxxxxxxxxxxxxxxxx (Jason Bayer)]

I have a client who is using Microsoft Outlook (on XP) to send large 
batches of e-mails to his clients using the Mail Merge function of 
Outlook.  When doing this Outlook creates an individual e-mail for 
each person on his list and adds these to the Outbox.  He may have a 
list of 50-100 e-mails in his Outbox waiting to be sent.  However, 
when he sends the messages they will process up to a point and then 
he will get an error message that reads:

The message could not be sent because one of the recipients was 
rejected by the server.  Server response: '550 relaying mail to 
<example: mac.com> is not allowed'.

Once he receives the error message, he can no longer send batches of 
e-mail.  If he clicks the Send/Receive button to send again it tries 
to send the remaining messages but he will always get the 550 error. 
I must then restart post.office for him to resume his batch sending.

I'm running Post.Office v. 3.5.3 release 714 on a PowerMac G3 450 w/ 
640MB - Mac OS X Server v .10.1.4

I not really sure where to start in troubleshooting this issue.  Any 
help would be greatly appreciated!

Jason Bayer
Architechnology, Inc.
11126 Kenwood Road
Cincinnati, OH  45242
513-985-9100
jason@xxxxxxxxxxxxxxxxxxx

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