Most old versions of
sendmail run only basic SMTP defined in RFC821.
In 1993 that service was extended by RFC1425 (obsoleted by RFC1869)
to become Extended SMTP (ESMTP). Beginning with V8
sendmail, you can enable a delivery agent to use
ESMTP by specifying the
F=a delivery agent flag.
This causes sendmail to first try to use the
extended form of the HELO command, called EHLO. If that fails to be
acknowledged as OK, sendmail tries again with
nonextended SMTP. If the initial SMTP-server greeting includes a line
containing the word ESMTP, the
F=a delivery agent
flag is assumed. (See also the
F=2 flag, F=2.)