VERP/Implementation

Steps
 * 1) travelwiki.wm.o sends a message to a user
 * 2) message envelope sender is bounce-travelwiki-f8h3hj4kh@wm.o
 * 3) destination address is sam@cats.com
 * 4) message goes out via mchenry.wm.o
 * 5) mx.cats.com says "bad address"
 * 6) mchenry.wm.o now has a bounce message queued, destined for bounce-travelwiki-f8h3hj4kh@wm.o
 * 7) mchenry.wm.o exim sees it has a local user bounce-travelwiki
 * 8) The handoff to the wiki's bounce-processor :
 * 9) could be mchenry-->travelwiki_webserver SMTP socket on some TCP port ( means we need a new daemon listening on the webserver) - not great
 * 10) could be mchenry-->travelwiki_webserver HTTP socket ( mediawiki is already listening there ) - better
 * 11) chenry could host enough mw code to run the bounce processing script, and talk directly to the database ( risky in the case of there being several different wikis to deal with) - not great
 * 12) maintaining a shared incoming bounce message db that wikis could access (maybe better from a security perspective because the mailserver doesn't need access to the wiki databases ) - better for security
 * 13) mailserver writes to a local mbox, and we run a POP/IMAP server on the mailserver, and the wikis could poll it.