CentOS 5.x | SendMail

I'd like to better understand how milter hand-offs work.

We a remote MTA initiates an SMTP session, my understanding is that Sendmail hands the message data off to the milter during that session. Where/how is this message data stored? Is this all done in memory? Or is there a "queue" area where message data for milter is stored for pickup by the milter?

If it helps, let's say I'm calling a milter like this:

INPUT_MAIL_FILTER(`foo', `S=unix:/var/lib/foo/foo.sock, F=T, T=C:5m;S:3m;R:5m;E:5m')dnl