Every now and them of of my users report that they are unable to send messages from their blackberry. This happens suddenly and without anything changing. How can I troubleshoot this? The only clue I have is the following event in the event log:
Event ID: 20000
Source: Blackberry messaging agent
Data: {email address} Send() failed: ERR_SUBMIT_MAIL, Tag=141170

The best thing to do is when they send a message out that fails, get them to open up the failed message and above the subject line you will see an error message, i.e Transaction Error, Failure at Service, Service Blocked

This should point you in the right direction by searching whatever error you get on the forum. I avoid Event logs where possible, they're normally full of crap as far as BES is concerned!

you could also use the messageflow.exe command from the besreskit.