Suppose that you are creating a service today. What can you reasonably and sensibly use email for, given modern email principles?

My answer is that the only real use of email today is as a notification system: you send your users email to tell them that there is something waiting for them on your site (whether that is a message, a state change, or whatever). This is a bit extreme, but I think it's the necessary end result of the limits.

Since modern email is not reliable, you can't make email the only that you tell users about something; the chance that it will get rejected, dropped, or filtered is just too great. (And when this happens, it will not be the fault of your users for not carefully making sure that your email bypasses all filtering.)

So you need to give users a way to get at this information, such as through your website. Since you don't want to train your users to trust email, you don't want to send full copies of information by email; instead you want users to read it off your site using some captive communication system (one that is not susceptible to various sorts of spoofing attacks).

This winds up with the conclusion of 'email as notification system'. You keep the real information inside your system, and only use email to let users know about the existence of new messages et al. If a user's email system drops some messages, there's no big loss; the user will find out about these new things the next time they wind up on your website itself.

(It may be useful to aggregate these notification emails, so that the user is not barraged with a storm of them.)