It definitely seems possible. Might be a bit hacky if it intercepts the normal email processing ?
The big problem is with the checks to actually send it later or not. Every existing notification would need an additional check function. Of course if only treating the subscription and mentions it's easier.
And then there's the problem of notification (non) duplication. Right now if an email is sent (when following), you won't get any new notification until you read the topic. The problem if a message is posted, then deleted, and some time after another message is posted, you probably won't get any email at all, and will never get any until you read that discussion. And you won't know you have to read the discussion to get notifications again because the email was suppressed...
The issue is not specific to this feature. It can already happen in some edge cases. Currently I think if the last post of a discussion is deleted, visiting the discussion won't reset the notification block for you. Somebody needs to publish a new post and you must read it in order for notifications to come again. Not sure anymore of the details if that's it but there's a situation of the like I encountered once when testing.
