I suppose it would be trivial to simply block or severely throttle high-volume SMTP traffic?

Additionally, this will make it look like you're a spammer.

You could block SMTP on almost all end user networks with no one noticing. 99.9+% of email client submission is HTTPS.

	▲	bauruine 14 hours ago \| parent \| next [-]
		I doubt it's that much but with the same logic you could also ban HN, SSH and basically any protocol thats not https "with no one noticing" because 99.9+% doesnt use it.
	▲	ranger_danger a day ago \| parent \| prev [-]
		Source: