SMTP is the protocol that mail servers use to send messages between each other, both sending and receiving. POP3/IMAP are protocols that your email client will use between itself and the mail server which allow you to view messages.

edit: I should note that you're correct in that SMTP is used by the client to relay email messages to the server also.

Thanks for that. So what exactly would you be able to use this for?

For one example, this would certainly take care of a lot of the work necessary for creating a disposable email service.

One existing use-case is Facebook's 'reply to this email to post a comment' feature.

Looks like it could be used in an application that needs to send email without relying on a platform-specific SMTP server or external service.

edit: I'm wrong, listen to this guy ->

It's actually for an application that needs to receive an email.

If you run your own mail server, you can usually configure it to forward mail over SMTP to a specific port, rather than delivering it to a mailbox for a client to pick up using IMAP. If you send the mail (using SMTP) to inbox.py, you'll get callbacks as soon as the mail is received, without having to worry about mailbox management and the complexity of an IMAP client.

I think it could probably be called "outbox" instead -- it might make a bit more sense?

This library is for receiving email, new email that you get, is put in your inbox. Email that you are sending goes in your outbox before it is sent. So I think it is named correctly. :)