There is a user preference for the default watch state. It's an opt-in user preference at the time of posting. Run this query to set the preference to email:
Code:
UPDATE xf_user_option
SET default_watch_state = 'watch_email';
Run this query to enable email alerts for all currently watched threads:
Code:
UPDATE xf_thread_watch
SET email_subscribe = 1;
Here is a query if you wish to enable watching + email for all existing threads (including threads that aren't even watched right now):
Code:
INSERT INTO xf_thread_watch (user_id, thread_id, email_subscribe)
SELECT user_id, thread_id, 1
FROM xf_thread
ON DUPLICATE KEY UPDATE
email_subscribe = VALUES(email_subscribe);