In the past few days, the entry notification feature has stopped working on my blog.

I've verified with several intended recipients that the mail was not received (tried sending it twice), though an entry notification I sent just 5 days ago was sent successfully.

I am using MT 4.01 and have made no adjustments to my blog settings or server settings in the past couple of weeks.

Any suggestions?

I can't help much with the notifications themselves if they're broken, as I don't use them, but have you checked that this isn't possibly a problem with the actual mail server? Messages could be stuck in queue, for one thing.

I think that's what happened -- they were stuck in a queue on my server. I received multiple copies of the notification this morning.