SMTP

SMTP is a member of the TCP/IP suite of protocols that governs the exchange of
electronic mail between message transfer agents.

Simple Mail Transfer Protocol - The original method of transmitting and receiving email on the internet. Still sometimes used for transmitting, but has been widely replaced by- POP3- and IMAP.

Sources:

Dictionary of Computer and Internet Terms (Vol. 1) John C. Rigdon, 2016 – 1471 c. – 1184