Well, as you see....
POP3 is a method of storing and receiving email.
SMTP is a method of sending mail.
When you send an email to your friend, your compuer sends the message to an SMTP server. The problem, however, with SMTP servers is the fact that spam can be routed through them if they are not shielded.
For example, if I wanted to spam someone, I would not do it through my ISP's account, because I would lose that account. So, instead, I might (no I am not a spammer) send it through someone else's server. This worked/s because older SMTP serevrs did not require authentication.
However, nowadays, if you run an SMTP server that is an "open relay," or non-authenticating server, you will surely be blacklisted. (See MAPS -
http://mail-abuse.org/.)
POP3 has always been authenticated. When a client (ie. MSOE, Eudora) connects to a POP3 server, they ive their login details and then request the number of messages in the server. It then downloads them from the server, and deletes them. (which is done to save space and to make the process easier.)
I hope this was a useful outline of email systems,
-[Unknown]
Author