Earlier quoted context omitted.
It is possible to encrypt SMTP connections with standard SSL/TLS technology. FastMail has been using opportunistic encryption on their incoming and outgoing SMTP servers for years. If you send an email to another service that does opportunistic encryption, and if both the sender and recipient uses SSL to access their mailboxes (as FastMail requires), the email will never be transmitted in plain text over the Internet…
The problem with such opportunistic encryption, is that you could insert a man in the middle which basically intercepts the traffic and modifies the handshake to exclude the STARTTLS extension. With opportunistic SMTP encryption this will cause things to proceed in plain text. The sinister thing about this is that e-mails still flow, so it still works.
( http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/produc... for anyone who wants details… )