Exchange 5.5 refusing external mail with 5.7.1

Recently our Exchange 5.5 residing on a Win2K server has started rejecting some email from outside with the error "5.7.1 Unable to relay for memyself@mycompany.com".  This is apparently only a problem with certain recipients and from a few specific companies, but I'm not sure where to look for the problem.  I've disabled relaying on my server to prevent spam.  Has anybody else had this problem and found a solution?
krcyberscottsAsked:
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David WilhoitSenior Consultant, ExchangeCommented:
post a copy of an NDR here.

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krcyberscottsAuthor Commented:
From: Mail Administrator [mailto:Postmaster@apples.com]
 Sent: Monday, November 17, 2003 12:11 PM
 To: somebody@oranges.com
 Subject: Mail System Error - Returned Mail
 This Message was undeliverable due to the following reason:

 Each of the following recipients was rejected by a remote mail
 server. The reasons given by the server are included to help you
 determine why each recipient was rejected.

 Recipient: <memyself@mycompany.org>
 Reason:    5.7.1 Unable to relay for kscott@cola.org


 Please reply to Postmaster@apples.com
 if you feel this message to be in error.

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Looking over the NDR I noticed that the server name is the company's old name -- different from the account that is handling the email.  Could the problem be on their end?
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David WilhoitSenior Consultant, ExchangeCommented:
If you have contact info on these few recipients, delete these contacts, and then rebuild them with new info from the latest emails you've received from them. i'd bet this resolves your problem....at least i'm hoping :)

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David WilhoitSenior Consultant, ExchangeCommented:
Did this work?? Just so everyone knows....

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