http://www.ietf.org/rfc/rfc2645.txt
With the spread of low-cost computer systems and Internet connectivity, the demand for local mail servers has been rising. Many people now want to operate a mail server on a system which has only an intermittent connection to a service provider. If the system has a static IP address, the ESMTP ETRN command [ETRN] can be used. However, systems with dynamic IP addresses (which are very common with low-cost connections) have no widely-deployed solution.

This memo proposes a new service, On-Demand Mail Relay (ODMR), which is a profile of SMTP [SMTP, ESMTP], providing for a secure, extensible, easy to implement approach to the problem.