I have been thinking about this some time... exactly 5 minutes, and I've thought that all e-mail notifications could be sent when the member is offline, because if I'm currently signed in the board and using it, I'll know when someone replied to my posts.
This would be done by adding a third option when asked
Enable Email Notification? [ Yes | Only when I'm offline | No ].
Look up the status of the user won't load the server, I think, and it will nicely decrease the SMTP-traffic.
Hope you like the idea.