Reminder that POP and IMAP are protocols, and nothing stops a code forge—or any other website—from exposing the internal messaging/notification system to users as a service on the standard IMAP ports; no one is ever required to set up a bridge/relay that sends outgoing messages to, say, the user's Fastmail/Runbox/Proton/whatever inbox. You can just let the user point their IMAP client to _your_ servers, authenticate with their username and password, and fetch the contents of notifications that way. You don't have to implement server-to-server federation typically associated with email (for incoming messages), and you don't have to worry about deliverability for outgoing mail.