[forkqueue]

Actually, not all email passes in plain text - a decent chunk (although doubtless a minority) uses SMTP over TLS.

Many servers have it configured, and if it's available on the destination almost all MTAs will use it to send mail to other servers, even if they don't support receipt of mail in this way.

[AndrewDucker]

Gmail uses TLS to encrypt SMTP if you're using a client app.