We have been having a consistent problem with Forum not sending emails. While playing around with various settings we've discovered that under the Site Administration menu if we go down the Modules/Activities/ManageActivities and set Journal to Hide, then the Forum mail function works. However, when Journal is set to Show then Forum mail is no longer sent (though it appears to be backlogged). Once that Journal setting is again set to Hide, then the mail that has been accumulating is sent.
It seems to me that this problem developed after an upgrade (but I do not know from or to what versions.) Has anyone else encountered this? How can it be corrected?