TLS Is a No-Brainer For Mail Servers
Opinion: There's no reason not to implement encryption between servers, even if it seems you're addressing a problem that doesn't exist.Encryption has been an important feature of many Internet protocols for some time now. Transfers between mail servers havent historically been protected, but there is a protocol available to encrypt them, and everyone should now look to implement it. TLS (Transport Layer Security) is easy to use. Of course, how easy it is depends on the specifics of each server, but generally its little more than clicking a checkbox in the servers administrative console. You do need to get a certificate, but you dont necessarily need to have one held by a certificate authority. You can generate your own certificate for free, and in many ways this works just as well. The end result is that mail transfers on the Internet between servers will be encrypted, as opposed to the default of clear text. The protocol is set up to request an encrypted transfer and fall back to unencrypted if the recipient doesnt support encryption.
We know its easy to do, and as far as I can tell theres no downside. This is why secure managed e-mail provider Postini has announced that it will support TLS (which they call "Postini Auto-Encryption") for all outside mail interaction with their e-mail boundary service Perimeter Manager Enterprise Edition 5.2. Postini claims to be the fourth-largest e-mail provider, so this move should increase the amount of TLS in use on the Internet considerably.