When I attempt a reply to one particular email address, I receive an error, "Message included an invalid address". This is happening only with one email address only, and has been going on for about 1 week.
Is it from another Blackberry user? He might have designated an invalid email address on BIS as the From/Reply to address. Just a guess but if so, you would receive his emails and not be able to reply to them. Go the From field in the received email and press Q to show the address. Is it correct?
Just verify the address with the person you are emailing. Sounds like a bad address could be the cause. I had this happen to me and it turned out to be a comma in the address in error and should have been a dot!