defer setting of the
DeliveryMode option (DeliveryMode)
is intended for sites that do not have a continuous connection to the
Internet (specifically, dial-on-demand sites). For such sites, mail
should be placed in the queue without interacting with the Internet
(which is likely unavailable). Then, when the Internet connection is
made, a normal queue run will deliver the mail.
Some database maps (such as those that look up hosts and possibly
those that log messages with syslog) should
probably not be used when sendmail is running in
defer mode. This -D database
switch can also be used with a few database-map types. When it is, it
advises those types to not operate when sendmail
is in defer mode.