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.)