Simple Mail Transfer Protocol (SMTP) is a messaging protocol which controls how email transmissions are performed between computers in Transmission Control Protocol/Internet Protocol (TCP/IP) networks.
SMTP is a server-to-server protocol and only supports text. SMTP traffic is both visible and easily readable. The default port for SMTP traffic is 25.
Since IPv6 assigns entire network ranges to individual devices, SMTP often blacklists at the network level, instead of the individual IP level.