NO sends to certain domains

ExchangeServer 5.5/SP4:
Mail server has been fine until 2 weeks ago. Then started gettin:
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The following recipient(s) could not be reached:

name@domain.com on 4/12/2002 12:51 PM Unable to deliver the message due to a communications failure
The MTS-ID of the original message is: c=US;a= ;p=MYDOMAIN;l=MAILSERVER-011413164856X-5
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DNS entries are fine in NT4/SP6a. No changes have been made to OS or Exchange. All service running (viewing Services). Re-applied Exchange SP4 and NT/SP6 andSP6a. Still same problem. Now there are about SIX domains that bounce back.

Is this a PR_RESPONSIBILITY issue? Is this a corrupt Exchange file?

HELP!!!