http://www.simplemachines.org/community/index.php?topic=11898.0
It seems that he has problems to receive messages notification not only on every SMF boards, but here also.
I searched a bit but did not find anybody how has reported a similar problem. Maybe an isolated case?
Could be his email provider is blocking the email for some reason? Marking as junk/spam?
-[Unknown]
He receives sometimes the notification email, sometimes he doesn't.
So, it could be his email provider blocking the notification mails :D
Remember also that you do not recieve notifications of your own posts.
-[Unknown]