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Outside email getting blocked. Error Transferring to; SMTP Protocol retured a permanent error 550 relay not permitted

We have a Windows SBS 2003 server that appears to be blocking one sending domain. The delivery failure report they receive (the sending email address) gives the following reason for the failure:
Error transferring to; SMTP Protocol Returned a Permanent Error 550 relay not permitted.

Does anyone know what could be causing these bounce backs from this domain.
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Failed to mention that we are using Exchange as the email server. Thanks

This is MS Access Question area,

please post a request here to move  this Q to Outlook or Exchange


Check the article above.
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Sorry for not providing the below information originally.

We receive email directly via SMTP and do not use a POP3 connector. Our MX record is pointing to our firewall which then port forwards all traffic to our Exchange server (there are no email filtration settings on the firewall). We are using Symantec Antivirus for Exchange however the senders domain has been white listed. I have been told that these two users have been sending email back and forth for some time and the issue is just now starting to happen. This makes me think it is something on their end but I am unsure how to guarantee that is the case.

Thanks again.
As the recipient your hands are tied. The sending site has to prove that the message is being delivered and rejected by your server. If they cannot do that, then the problem is with them.

As I stated above, this is not an Exchange message, so if your firewall is just port forwarding then there is nothing else generating the message.

Ensure that Symantec isn't getting in the way - I have seen that product cause too many problems in the past.

The sender using Lotus Notes. We attempted sending an email from a third client and it went through. The problem is on their end.