Most MTA’s should offer Opportunistic TLS by default
I think that the time has come for most SMTP MTA servers to offer STARTTLS session protection by default. I see two reasons for doing this. Firstly, it takes a short amount of extra time and a little more CPU horsepower and that’s a resource that spammers cannot control. Secondly, opportunistic TLS brings email security a little more in line with the security model that most users expect.
Spammers
The majority of spammers out there are relying on stealing CPU time on machines that they don’t own. I don’t see them moving to TLS at the client side anytime soon. On the other hand legitimate email senders usually aren’t sending mail in such bulk that the cost of encrypting the session would be an onorous penalty. The practical end result of this would be differentiation of mail at the inbox. We would get mail from servers that used TLS to encrypt the session and mail from servers that didn’t. Assuming that the MTA server flagged the mail on this axis by adding a header, the end result is a hook for a statistical spam filter to use.
Users expectation
The second advantage would be a little added security in email during the transport from client MTA to server MTA. If everyone adopted opportunistic TLS encryption of the wire then sending email would better approximate the users expectations for security. Compared to physical mail email without TLS is like sending a postcard. No one sends postcards where security is a requirement because it’s obvious that everyone between the point where you drop the mail in the postbox and the point of delivery can just read the mail. Most users don’t expect that this is the case with email right now.
The advantage of opportunisticaly encrypting the mail is that we have a situation that we can grow into. If some server doesn’t do TLS in transport the mail still gets delivered.