sendmail, 4th Edition

F=0 (zero)

F= delivery agent flags:Turn off MX lookups for delivery agent V8.8 and later

During the delivery phase of a message, sendmail looks up the destination hostname with DNS and (possibly) redirects delivery to MX hosts, if present. One way (but not the best way) to suppress that MX lookup is to surround the destination hostname with square brackets:

% /usr/ucb/mail -v user@\[mail.us.edu\]

Note that the square brackets are retained as part of the SMTP envelope:

RCPT To:<user@[mail.us.edu]>           ← square brackets retained

The F=0 delivery agent flag is another way to suppress MX lookups. To illustrate, consider using this delivery agent flag with FEATURE(nullclient) (FEATURE(nullclient) on page 637):

FEATURE(`nullclient',`mail.us.edu')

Here, all mail will be forwarded to mail.us.edu. To suppress MX lookups, we could surround the address with square brackets:

FEATURE(`nullclient',`[mail.us.edu]')

But this is unattractive and unnecessary. Instead, we use the F=0 delivery agent flag to achieve the same MX suppression effect:

define(`SMTP_MAILER_FLAGS', `0')        ← prior to V8.10
define(`RELAY_MAILER_FLAGS', `0')       ← prior to V8.10

MODIFY_MAILER_FLAGS(`SMTP', `+0')       ← V8.10 and later
MODIFY_MAILER_FLAGS(`RELAY', `+0')      ← V8.10 and later

FEATURE(`nullclient',`mail.us.edu')

Note that the F=0 delivery agent flag is suitable only for configurations such as nullclient. It can be extremely dangerous to use with any other delivery agents because it will cause necessary MX lookups to be skipped.