Three protocols are involved in email communication: SMTP (Simple Mail Transfer Protocol) for sending messages and POP (Post Office Protocol) and IMAP (Internet Message Access Protocol) for receiving messages. The latter two protocols are often represented with their current version numbers: POP3 and IMAP4. If the communication is encrypted, using SSL/TLS, for example, the names used are SMTPS, POP3S, and IMAPS, where the added S means Secure.
The sender’s mail client hands over the message to an SMTP server. This ...
