Mail protocol (Email)

Mail sending and receiving process-STMP, POP, IMAP, MIME
email sending protocol is a "push"-based protocol, mainly including SMTP;
mail receiving protocol is a "pull"-based protocol, mainly including POP Protocol and IMAP protocol,
before formally introducing these protocols, we first give the architecture of mail sending and receiving:

https://www.cnblogs.com/gd-luojialin/p/10963030.html