The 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.