you mean that means of communication where spam problems effectively forced a centralized infrastructure provided by a few providers and that now is relegated to mostly being used as a poor man's notification service?

running your own smtp server these days is painful.

Not that painful (speaking from experience of running one).

It's somewhat painful, but not because of spam. The pain all comes from jumping through hoops for the centralized providers (and then discovering that no matter how many hoops you jump through, it's never enough).

And see the subsequent flocking to APNS, its child iMessage, Firebase Messaging, and Facebook Messenger.

Email is obsolete.