Lesson 1: Don't Tarpit Yourself

1-1 connection mapping resulted in HUGE numbers of concurrent connections to production mailserver.
New connections were being made far faster than they were being released.
Solution: Spool and classify messages entirely before forwarding to production server. Forward to internal SMTP server last.
Neat Side Effect: Better scaling. Several TarProxies on cheap servers can proxy to the same production server.

1-1 connection mapping resulted in HUGE numbers of concurrent connections to production mailserver.