Asahi Net : Internet Software Guide
POP before SMTP
What is POP before SMTP?
POP before SMTP is where authentication takes place when you get (receive) email, allowing you to then send your email. After the authentication, there is a 10 minute period in which sending of email is possible.
Some email software, like Outlook Express for Windows, will automatically try to send a composed email first, before receiving, and this will cause a Sending error. In this case, close the error window, and try to receive and send again.