Exchange 2003 SP2 Event ID 7010

I have an Exchange 2003 SP2 server that handles our companies internal and external email.  We have always been able to send/receive external email without any issues.  Recently I have found out from other staff that there are 2 domains for which email is not showing up from.  They are able to send mail to these 2 domains fine but when the other side sends mail back it gets lost.  I've done some research with the diagnostic logging and I can see that the mail servers try connecting but I am getting the following error:

This is an SMTP protocol log for virtual server ID 1, connection #18. The client at "" sent a "xexch50" command, and the SMTP server responded with "504 Need to authenticate first  ". The full command sent was "xexch50 2112 2".  This will probably cause the connection to fail.

The mail never gets delivered to our server and the sender never gets a NDR.  It seems as the two domains have never been able to send mail to our Exchange server.  All other email seems to be flowing fine.  Any ideas would be greatly appreciated.
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How to troubleshoot the "504 need to authenticate first" SMTP protocol error
noemaineAuthor Commented:
I have gone through the knowledgebase article and the posts on event_id. Niether have rectified the problem.  I'm having no issues sending mail to any domain; just receiving mail from 2.  It's very strange.  I have also checked our firewall setting to make sure SMTP commands are not being blocked.  
noemaineAuthor Commented:
The problem has been resolved.  The ISP hosting the domain was not forwarding the emails onto our Exchange server.  Instead they were collecting them and marking them as delivered.  Very strange but they have since fixed the issue and emails are coming through now.
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