VERP/Implementation

Steps
 * 1) wiki.wm.o sends a message to a user
 * 2) message envelope sender is bounce-wiki-f8h3hj4kh@wm.o
 * 3) destination address is sam@cats.com
 * 4) message goes out via mx.wm.o, which attempts to deliver to mx.cats.com
 * 5) mx.cats.com says "sam@cats.com is not a valid address"
 * 6) mx.wm.o queues a bounce message, destined for bounce-wiki-f8h3hj4kh@wm.o
 * 7) mx.wm.o's MTA accepts mail for wm.o and does a lookup for local user bounce-wiki
 * 8) this is where the handoff to the wiki's bounce-processor happens, which could take many forms:
 * 9) mx.wm.o-->wiki.wm.o:SMTP
 * 10) wiki.wm.o could receive via its main MTA, which passes back to a mw script
 * 11) wiki.wm.o could run a new daemon, all mw code, that handles SMTP directly
 * 12) mx.wm.o-->wiki.wm.o:HTTP with mw bounce processing code at that URL
 * 13) mx.wm.o could host enough mw code for a local bounce processing script which talks directly to wiki db
 * 14) mx.wm.o could queue to a bounce message db that wikis could access
 * 15) mx.wm.o could write to a local mbox, and runs a POP/IMAP server for wikis to poll