It can only withstand so much downtime though. What if you don't have access to your server for a week? Heck, even a couple days can screw things up. So much email is time-sensitive. From transit delay notifications to message notifications and bank alerts... to actual humans that matter in your life trying to get something from you by tomorrow thinking you'll get their emails as usual. Maybe it's just me but I just don't see how "oh it's chill if my emails get randomly delayed for a day or two" would fly in today's world. Intentionally going off the grid after you've prepared for it is one thing, but having your email randomly go out without any warning in a potentially awfully inopportune time is quite something else...
> What if you don't have access to your server for a week?
Then you've chosen a bad service provider(s) to host/colocate/peer with.
Actually, you should notice this situation pretty much immediately you're online - because most likely your IMAP server would be down - and every single email client I've used had started to display warnings.
If you have more complex setup with separate MTA so your IMAPd may be online while your STMP service's unreachable, then you should set up monitoring system and deliver alerts either using local delivery or any out-of-band mechanisms (like SMS).
Also, you can set up a backup MX. Or two. Or more. Email has failover/HA since forever.
