550 5.0.0 User Unknown?

One of my customers is using SBS 2003 with exchange, when I send an e-mail, the mail is immediately rejected with the error: The following recipient(s) could not be reached: 550 5.0.0 User Unknown. If I send via Hotmail however, the message is delivered? I also get a different problem from another customer who is also using SBS 2003 with exchange. I can e-mail them, but if they try to e-mail me they get: The following recipient(s) could not be reached:550 5.7.1 Unable to relay? For your additional info, I'm using outlook 2003 via a pop3 account. All help greatfully recieved...
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ProtecUKConnect With a Mentor Author Commented:
Both of the above issues have now been successfully resolved. The issues were being caused by the clients e-mail service provider. The clients were initiallly using POP3 accounts but then installed exchange server. When the MX records were ammended to point directly at the exchange server, not all of the old settings were removed from the e-mail service providers servers. Once this was done, all problems ceased. I hope that this helps others who face a similar issue.
get rid of POP3 account and use outlook in its native mode. Are you able to connect to Exchange using OWA?
ProtecUKAuthor Commented:
Hi Meugen, yes I am able to connect to the clients OWA with no problems and can send and recieve fine. If I try to send to the first customer via OWA from another customer, the mail is also received fine. The only time the mail is returned is if I send it via Outlook from my own e-mail address. I don't have a server, so I'm using outlook 2003 with a pop3 account provided by Easyspace from my domain name. Many thanks in advance..
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