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 human-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.