Some mail from one PC in domain is rejected

In an existing Windows 2003 domain where I've been replacing Win XP Pc's (w/Outlook 2003) with Win7 Pcs (w/Outlook 2007), one of the new pc's with Win7 & Outlook 2007 gets about five email rejects a day. A co-worker next to her with the exact same PC & Software sends to the same recipients perfectly. They are both in the same domain, server and everything else.

What could cause her email to be sporaticly rejected when her co-worker can send to the same person perfectly?
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Tony GiangrecoAsked:
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What is the mail reject code? Eg: 5.7.1? or post the reject leter

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Tony GiangrecoAuthor Commented:
The reject was #5.1.2. I contacted the recipient and they have a Domino mail server. Even though it's old, they mentioned they send and receive a ton of mail every day.
are you using exchange or another email server for outgoing mail?
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5.1.2 error says bad destination host.

This sender either can not resolve the DNS query to the destination, or is filtered at the destination.
Tony GiangrecoAuthor Commented:
Exchange 2003 is on the server with Outlook 2007 on the Pc. The destination runs Domino.

On the sending side, her co-worker can send to the same person at the destinstion perfectly. They have idential Pc's with Identical software. I set both up the same week.

Win7 Pro, Symantec Endpoint 11.0.6, Windows 2003 domain, Exchange 2003...
is the mail setup using pop/smtp or the microsoft exchange (in outlook)

because if its exchange the server should be sending the mail.
if its pop, it might not be setup correctly

does the user have any other email accounts setup?
Tony GiangrecoAuthor Commented:
Her Outlook profile only has one entry which is the Exchange mailbox. She only works in the facility so she is directly connected the to onsite exchange server.

I noticed in Exchange, the 1st two letters of her name were capitalized which is unusual for this company. I added an identical email address for her using all lower case and I set it as her primary SMTP address. I will find out on Monday if this makes any difference.
Maybe the client is looking outside your domain for your exchange server. What are the primary and sencondary DNS servers on the client. Is the client looking to the outside world, periodically, for DNS resolution back to the exchang server?
Tony GiangrecoAuthor Commented:
I updated multiple settings and this problem is resolved. I'm not sure which setting change actually corrected the problem, but mail is flowing properly now.
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