After upgrading to ActiveForums 4.2.7, some of my users are no longer receiving email notifications for forums that they were subscribed to prior to the upgrade. Additionally, other users that are subscribed to the forum do receive email notifications for the same forum.

It appears that the fix is to add the DNN "Registered Users" role to the forum and give it "subscribe" permissions. I'm not sure this is a viable long-term solution.

Scott,
Are these forums within a group? If so, do the group members have subscribe permission?

Yes, the forums are within a group and both of the group roles, Admin and Member, have subscribe permissions.