Mine gets caught because there is no authentication on sending emails in a POP setting (IIRC). So a retreive emails function does have a password (sent in the clear w/o SSL IIRC) and I can send emails from those accounts for the next 3-5-10 minutes. If it times out I have to retrieve mail again.
Because your email service might be down. Also if your internet connection gets interrupted, the outbox will appear and begin catching any email you're trying to send, and then hold it until you return to a connection.