You need a CA for TLS-secured comms between MTAs. Many MTAs are set up to do opportunistic encryption out of the box, but they won't be validating the certs they get, so there's no guarantees about who's got the private key.

Of course there's no need to rely on a central CA - it's not hard to run your own, and you can make it reasonably secure - say, a small ARM Linux board with passphrase-protected private keys on a removable SD card. Generating the keys in a secure way is perhaps a bigger problem - untrusted hardware RNGs, poor quality entropy after boot etc.

One can also keep a virtual machine CA on a hardware-encrypted USB device (IronKey, say, depending on your level of trust with Imation) - it's easy enough to bootstrap a tiny Linux distribuution from source with just OpenSSL and some utility scripts to issue & revoke certs.

You don't necessarily need a $5000 HSM solution to issue your own SSL certificates at this level.