http://www.ietf.org/rfc/rfc2852.txt
This memo defines a mechanism whereby a SMTP client can request, when transmitting a message to a SMTP server, that the server deliver the message within a prescribed period of time. A client making such a request also specifies the message handling which is to occur if the message cannot be delivered within the specified time period: either the message is to be returned as undeliverable with no further processing, or a "delayed" delivery status notification (DSN) [6] is to be issued.

This extension should not be viewed as a vehicle for requesting "priority" processing. A receiving SMTP server may assign whatever processing priority it wishes to a message transmitted with a Deliver By request. A Deliver By request serves to express a message's urgency and to provide an additional degree of determinancy in its processing. An indication of an urgent message's status within a given time period may be requested and will be honored. Moreover, the message may be withdrawn if not delivered within that time period.

A typical usage of this mechanism is to prevent delivery of a message beyond some future time of significance to the sender or recipient but not known by the MTAs handling the message. For instance, the sender may know that the message will be delivered as a page but does not consider the message to be sufficiently important as to warrant paging the recipient after business hours. In that case, the message could be marked such that delivery attempts are not made beyond 17:00. Another common usage arises when a sender wishes to be alerted to delivery delays. In this case, the sender can mark a message such that if it is not delivered within, say, 30 minutes, a "delayed" DSN is generated but delivery attempts are nonetheless continued. In this case the sender has been allowed to express a preference for when they would like to learn of delivery problems.