On-Demand Mail Relay

On-Demand Mail Relay (ODMR) is an SMTP extension standardized in RFC 2645 that allows e-mail to be relayed to the recipient after he has been authenticated. It uses the extended SMTP command ATRN, similar to the ETRN command but available for dynamically assigned IP addresses.

ODMR works in the following way: to check his inbound mail, the customer issues EHLO and AUTH commands to the ODMR service, followed by ATRN command (Authenticated TuRN); after a successful ATRN command the ODMR server begins to act as an SMTP client and starts to send all messages directed to a customer's domain (or domains) using SMTP protocol, via the same TCP connection. Since the connection is initiated by the customer, this protocol works well for machines having dynamically assigned IPs and/or are firewall protected (i.e. your ISP may block incoming connection on port 25 or may use NAT). Only the ODMR server, referred as the provider, must listen to SMTP connections on a fixed IP address.