[SwellJoe]

I think the only thing this particular protocol should be concerned with is reliably providing a method for communicating between client and server. Most spam and malicious mail filtering should happen as early in the process as possible, hopefully during the initial connection.

There is a a protocol named Sieve ( http://www.ietf.org/rfc/rfc5228.txt ) which provides for delivery stage filtering rules. It is similar in capability and usage to procmail, but formalized and somewhat more modern (and not capable of running arbitrary system commands), and supported by some modern IMAP/POP servers, like Dovecot. Presumably if JMAP gets integrated into those servers, Sieve would also be available.

And, I would hope there wouldn't be an IMAP/JMAP translation layer, but instead servers would implement JMAP directly. Though I guess in the short term there might be some sort of proxy.

[gecko]

Sieve is the native filtering engine used by FastMail, so I'd say it integrates quite well with JMAP, from my experience. :)