This was all working fine last week...
...we restarted the Exchange services at the weekend, and since then our event log on the EX server has filled up with 7010 messages - 5126.96.36.199 Unable to relay for
The BES event log is full of 10000 events, basically reporting the same issue - that the mail has not been sent with a 550 5.7.1 error.
It does appear to be more of an Exchange isue, but as everything else appears to be working, I can't see why BES is unable to send it's message out?!