The Precedence: header, when the QueueSortOrder option (QueueSortOrder) is appropriately set, is used internally by sendmail to order the processing of messages in its queue. A full description of the possible field values for this header is given in Section 25.10.1. The effect of those values on ordering the queue is described in Section 11.7.

The Precedence: header should never be declared as an H line in the configuration file. However, P precedence lines should be declared in that file.