Quite the contrary: most installs I've seen have a "remote" dedicated SMTP server which is actually on local network (not significantly slower than mail()), and

Quite the contrary: most installs I've seen have a "remote" dedicated SMTP server which is actually on local network (not significantly slower than mail()), and does other useful stuff like DKIM and SPF (which, with appropriate setup, has the added bonus that rogue app-server emails don't just appear out of nowhere, and at the very minimum have to pass through this server, giving you a place to debug). Of course, if you're making My First Blog Server on a shared host, then all of this is moot.