I've written a couple of websites that have user-to-user messaging, once even a dating site. I didn't use SMTP at all, and I'm surprised why you would consider

I've written a couple of websites that have user-to-user messaging, once even a dating site. I didn't use SMTP at all, and I'm surprised why you would consider it. Assume for the moment you created mailboxes for each user, assume you have two users "Alice" and "Bob". If Alice wanted to send a message to Bob you certainly could send an email to a local SMTP-server via an address like "Bob@mail.local". Then when Bob logged into your site you could poll that mailbox via IMAP/POP3 and retrieve the message. But really it would be far simpler to just query a database, avoiding the overhead of needing to speak IMAP(S)/POP3(S). Especially if your site is dynamic already and needs to use a database. Sure use outgoing SMTP to send an email to the real-address of a member "You've got a message from Alice".