What are POP and SMTP servers?

Post Office Protocol is the most common protocol used to retrieve e-mail from a mail server. Most e-mail applications (sometimes called an e-mail client) use the POP protocol, although some can use the newer IMAP (Internet Message Access Protocol). The newest version, POP3, can be used with or without SMTP (an e-mail sending protocol, stands for Simple Mail Transfer Protocol). IMAP servers are similar to POP servers, the only difference being they save the e-mail so they can be retrieved from multiple locations or multiple users.