Hi,
I thought I'd mention this since other people are bound to have this issue.
I'd noticed that no email notifications or catalog DR emails had been sent out after applying MP4 onto a Windows Master (Server 2003, SP1).
It turns out that a file called nbmail in the bin dir had been overwritten; what had actually caused the NBU emails to stop was the fact that the new file was just a file of commented out lines (and therefore did nothing).
You can probably uncomment out the required lines but I just replaced it with the pre-MP4 file (had MP3 previously).
Cheers,
Tim