The sendmail program can run in any of several modes, each of which determines its behavior. When sendmail first starts to run, it sets its mode based on the setting of its DeliveryMode option (DeliveryMode) and places the character representing that mode into this ${deliveryMode} macro. If sendmail is run with the -odi command-line switch, for example, this ${deliveryMode} macro is given the value i. Once the sendmail program is running, its delivery mode can be changed for a variety of reasons. When it starts to process the queue, for example, the mode is changed to d (for deliver).

One use for the ${deliveryMode} macro can be seen in the standard configuration file:

SBasic_check_relay
# check for deferred delivery mode
R$*                     $: < $&{deliveryMode} > $1
R< d > $*               $@ deferred

Here, the Basic_check_relay rule set is called to determine whether mail from the connecting host should be accepted. Because the hostname of the connecting host is not looked up with DNS when in deferred mode, many necessary policy checks should not be performed (such as access database lookups) because the true hostname might not be known. These rules cause those checks to be skipped when in deferred mode. Later, when the message is processed from the queue, the hostname will be looked up with DNS.

Because it is unlikely that the sendmail daemon will be run with DeliveryMode=d set in the configuration file, there is no need to prefix ${deliveryMode} with an ampersand in the first rule. We did so here because "good style" says to always use the ampersand.

The ${deliveryMode} macro is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail.