Tracking issue for VRFY support

[novedevo]

this one's a little trickier than some other commands. RFC declares that we can't just check syntax, we need to make sure that we actually have a user at that mailbox, so to support VRFY we already need to maintain a user database. In the initial versions, this could be a .txt file, or a Vec<String>, but if we want this to be a functional server we'll need something a little heavier.

Similar issues apply to EXPN, but if we just don't create any mailing lists we can trivially return for that one.

[novedevo]

if we do end up supporting EXPN, we need to add it to our EHLO response

[novedevo]

we also should put a config option to disable VRFY and EXPN for security reasons, returning 252