Encryption without a certification layer is (partly) useless
With the PRISM scandal, there has been some talk about encrypted communication systems. For instance, BitMessage is often introduced as an easy and secure message system, that would allow you to communicate with no possible eavesdropping. Apple is also making similar claims about their systems iMessage and FaceTime.
This is a good time to remind this: without direct contact or a certification layer, encryption systems are not secure! Or at least, not as secure as you would expect, as they do allow some kind of eavesdropping.
Read more Encryption without a certification layer is (partly) useless
Looking for an SPF milter
For email extensions such as SPF, DKIM and DMARC, I think the most flexible and portable system is the milter protocol. Originally developed for Sendmail, it is now also supported by Postfix, and it allows to “plug” specific filters in the mail server without the hassle of the previous systems like SMTP proxies.