An MTA protocol defined by IETF RFC 2821. SMTP is the most commonly used MTA protocol.
Source(s):
NIST SP 800-45 Version 2
the primary protocol used to transfer electronic mail messages on the Internet.
Source(s):
NIST SP 1800-16B
under Simple Mail Transfer Protocol
from
NISTIR 7387
NIST SP 1800-16C
under Simple Mail Transfer Protocol
from
NISTIR 7387
NIST SP 1800-16D
from
NISTIR 7387
NISTIR 7250
under Simple Mail Transfer Protocol
NISTIR 7387
under Simple Mail Transfer Protocol