There are
times, however, when mail should not be sent until it is asked for. Consider the typical ISP.
Clients who connect over dial-up lines are not necessarily connected when mail arrives for
delivery to them. The F=% delivery agent flag has been added to prevent sendmail from
trying to discover if there is a connection.
The F=% delivery agent flag, when set, prevents immediate delivery to destination hosts.
Instead, sendmail queues all messages. Each destination host must then request delivery
using the ETRN command (?§11.8.2.6 on page 433) after connecting. One way a client can
give the ETRN command is by using the etrn.pl script supplied in the contrib subdirectory
of the source distribution.
The local administrator can also cause delivery to occur manually for specific clients with
any of the -qI, -qR, or -qS command-line switches (?§11.8.2.3 on page 431). Note that a
standard queue run (as with -q) will not send messages that have been deferred because of
this F=% delivery agent flag.
20.8.2 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.
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