We want to handle this syntax: username+mailbox@domain

Using this algorithm:

1. User-specified (e.g. by command line option to dbmail-smtp or in a Sieve script) non-existant mailboxes are always created.
2. Non-specified mailboxes go to INBOX, which is created if needed.
3. Externally specified mailboxes which go to an existing mailbox are respected, while non-existant mailboxes are checked:
   1. If in a list of approved boxes, such as Spam, Drafts, Sent, etc., the box is created.
   2. If not, it goes to INBOX, which is created if needed.

Status as of Feb 2006 is that the incoming mail envelopes are checked for a +mailbox spec, except in the case of domain or user catch alls (should we support this?), and passed along the delivery chain. When it's time to put the mail into the mailbox, we only test for mailbox existence, else it goes to INBOX (which will be created on the fly, if needed). TODO: Approved mailboxes list in the new DELIVERY section of dbmail.conf