I don't think you could use POP or IMAP, since these protocols are designed for a registered user to interact with a mail server - to connect, you'd first need

I don't think you could use POP or IMAP, since these protocols are designed for a registered user to interact with a mail server - to connect, you'd first need to know the user's userID and password on that server (and the userID isn't necessarily related to the e-mail address, even though it may be on Gmail). And if you got back an error, you wouldn't know whether it was the userID or the password that was invalid. SMTP can be used, however, since that protocol is designed to be able to send messages to a user on any mail server. (See my earlier reply for an explanation of how this can be done with SMTP: https://news.ycombinator.com/item?id=24834067 https://news.ycombinator.com/item?id=24834067)