There is one thing in forums that is annoying me when using mail notifications: When I turn on mail notifications and I am in the forum I can see that there is a new post on a watched thread for example. Therefore I do not need another mail. From my point of view that is going to be spam mail and I am deleting them because I have seen the notification already in the forum.
I would prefer an user option that can define a delay of a mail notification and do not send me one when I am currently logged in the forum and see the alert.
I can see it being server intensive if the check is ran every time an alert is processed but if it's ran at the first alert to calculate how long they have been active for then ran after that time period to check if they're still active it may not be quite so intensive.
Check if logged in -> Last active: 5 minutes ago -> Send alerts without e-mails for 10 minutes without checking -> Check after 10 mins, if active, calculate 15 minutes in advance and send alerts without e-mails. If not, proceed to send e-mails.
Of course it will mean that some e-mails will be delayed, but hey.