Ordinarily, outbound mail is dispatched as soon as it is handed to sendmail. 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 (Section 11.8.2.6) 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 with any of the -qI, -qR, or -qS command-line switches (Section 11.8.2.3). Note that a standard queue run (as with -q) will not send messages that have been deferred because of this F=% delivery agent flag.