Currently Synapse only has two options: All notifications are sent as email or no notifications are sent. This issue is to add fine grained settings, e.g. one might want to suppress mail from a certain person or Team.
We could add an option to send messages as a daily digest rather than one at a time.
Notification emails should have another 'footer' with a link taking the user to their notification settings page in Synapse (and prompting them to log in, if necessary).
Another option is for 'fine-grained' one-click unsubscribe footers, e.g. a notification might say "user xxx has asked to join team yyy" and the link could say "click here to turn off membership request notifications" or "click here to turn off messages regarding team yyy".