Checking email addresses

My friend Morbus writes that he would like some way to validate email addresses... Well, the SMTP specification defines the VRFY command, which will validate that a given mailbox exists on a given server... I've tried this on various servers (including my own) with mixed results. Some servers will return an OK for ANY address, some only for those with actual mailboxes, and some for any address that has a forward, mailbox, or route setup.

Your mileage may vary, but it's a start.