I had the exact same problem and after a lot of red herrings I've managed to fix it.
What put me onto this solution was that no messages were reaching the GWIA (GroupWise Internet Agent), suggesting that messages were being blocked before they reached it....
In my case it was McAfee VirusScan Enterprise causing the problem, I daresay that this problem will occur on other AV products too.
For McAfee :
Right click the systray icon and click 'virusscan console' double click the 'Access Protection' task
Look for port 25 in the ports column, highlight it and click 'edit'.
Under excluded process you will need to add beserver.exe
Click OK, and you should find your alerts will now send without being blocked.
Apologies to the mods if mentioning rivals products is frowned upon, sorry!