messages can be placed into the queue either intentionally or because
they could not be delivered immediately. Once they are in the queue,
two time periods come into play. First is the period of time that the
message should remain in the queue before a warning is issued to the
sender. Second is the total period of time that the message should
remain in the queue before it is bounced as a failed message.
Beginning with V8.7 sendmail, it is possible to
tailor these intervals on the basis of three distinct priorities of
mail. The new Priority: header tells
sendmail which priority a message possesses:
Here, pri can have one of three possible
values: urgent, normal, and
non-urgent. These values correspond directly to
the priorities specified by the Timeout.queuewarn
option (See this section) and
Timeout.queuereturn option (See this section):
Here, a Priority: header of
normal will cause the message containing it to
bounce after it has remained in the queue for two days.
The Priority: header should never be declared in
the configuration file.