phpbb_notification_method_email unnecessarily loads data of banned users.

First, banned user ids are fetched. Then all affected user rows are fetched and each row is checked whether the user is banned. Also, $user['user_type'] == USER_IGNORE rows are skipped.

a) Do not fetch users who are banned from the database. An array_diff() should do.
b) Do not fetch users with $user['user_type'] == USER_IGNORE. Either make sure such messages are never in the queue or ignore in the SQL query fetching users.