sendmail, 4th Edition

F=2

Force SMTP even if ESMTP is offered V8.12 and later

When sendmail connects to a remote site, it looks for a literal ESMTP string anywhere in the initial acceptance message:

220 wash.dc.gov ESMTP CommuniGate Pro 3.5.9

If the accepting site says that it can accept extended SMTP by including the ESMTP string, the local sendmail will answer with EHLO instead of HELO.

If you want to limit a particular delivery agent to nonextended SMTP, no matter what the receiving site says, you can do so by defining this F=2 delivery agent flag:

MODIFY_MAILER_FLAGS(`SMTP', `+2')              ← this must be
MAILER(`smtp')                                 ← before this

Here, we add the F=2 delivery agent flag to the smtp class of delivery agents. As with all modifications of delivery agent flags, the modification must precede the MAILER declaration of the delivery agent.

This F=2 flag is most useful with broken MTAs and firewalls. When sendmail connects to such a broken site, that site will print ESMTP in its greeting message. But that ESMTP is incorrect, and when sendmail sends EHLO, the broken site will reject the salutation and drop the connection.