Run the DB command to turn followed posts/replies/forums email notifications OFF for all users, including the defaults for new users (member #1). Users can turn on email of these notifications as they choose. Doing that should cut down on the sent mails, while still maintaining an expected forum functionality. new user emails and triggered password requests are always emailed.

Set all users forum email preferences to ubb.threads stock defaults
SQL Query
update ubbt_USER_PROFILE

set this to 0 for no emails of watched items. users can change their personal accounts back if they choose. you only need to do this once, since it will also change the status for the new user account defaults.

edit: i do not believe this has any effect on private message email notifications, or mail from the forum admin, such as direct and mass mail.