I am running Exchange 2007 on Windows 2008 Server R1.
Symantec SMSMSE 6.5 is provides spam protection.
Generally, there are no problems in receiving mail from thousands of senders.
The problem is with a few specific senders - that is specific companies/server/domains cannot send mail to any of our recipient addresses - no mail from any sender at those locations reach us..
Despite the addresses they send to are entirely correct, for instance a reply to our mail to them, they receive an error (and we can send to them with no problems).
In error messages there will be a peamble from their own server, ending with
550 5.1.1 <firstname.lastname@example.org>: Recipient address rejected: User unknown in relay recipient table
However, as mentioned the address they send to on our server is correct.
I have added the sender to our whitelist, for safety's sake, though the error is not spam related anyway.
Any suggstions on probable cause or how to pinpoint the cause?