Hi David

When maintenance mode is enabled cron is prevented from being run. Cron is used to send most emails including system emails.

When maintenance mode is disabled the cron will run and any notifications that were due to be sent will still be sent . The behaviour of notifications differs depending on whether legacy notifications are used where notifications are queued which Totara 12 uses

Also some notifications sit outside legacy and scheduled tasks so behaviour is not consistent - maybe redirecting to a catchall mail or a separate mailserver might be safer so you can see what emails are generated and forward only the ones you want to send.

regards