On-Demand Mail Relay
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ODMR (On-Demand Mail Relay) is an extension to the SMTP e-mail protocol. ODMR is defined in RFC 2645. It is similar to ETRN but works with dynamically assigned IPs.
The ODMR works in a very simple way: after connecting to the ODMR service EHLO and AUTH commands are issued, followed by ATRN command (Authenticated TuRN); after a successfull ATRN command the ODMR server begins to act as an SMTP client and stars to send all stored messages using SMTP protocol, using the same TCP connection; since the connection is initiated by the client, this protocol works well for machines having dynamically assigned IPs and/or firewall protected (i.e. your ISP may blocks incoming connection on port 25 or may use NAT, as done by Fastweb in Italy)
[edit] ODMR with SMTP server not supporting it natively
ODMR design allows you to use the ODMR protocol also on an SMTP server that doesn't supports it natively.
A second application can be used to do the initial ODRM protocol negotiation (EHLO, AUTH and ATRN); the this application must just transparently connect the ODRM service with the SMTP server and exchange data between them.
Follows a list of free applications implementing ODMR that way:
