Mail 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:

Priority: pri

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):

O Timeout.queuereturn.urgent=1d
O Timeout.queuereturn.normal=2d
O Timeout.queuereturn.non-urgent=4d

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.