Editing On-Demand Mail Relay

{{Short description|SMTP extension standardized in RFC 2645}}
{{refimprove|date=August 2011}}
'''On-Demand Mail Relay''' ('''ODMR''') is an [[Simple Mail Transfer Protocol|SMTP]] extension standardized in {{IETF RFC|2645}} that allows [[e-mail]] to be relayed to the recipients after they have been authenticated. It uses the extended SMTP command <code>ATRN</code>, similar to the <code>ETRN</code> command but available for dynamically assigned IP addresses.

ODMR works in the following way: to check their inbound mail, ''customers'' issue <code>EHLO</code> and <code>AUTH</code> commands to the ODMR service, followed by <code>ATRN</code> command (Authenticated TuRN); after a successful <code>ATRN</code> command the ODMR server begins to act as an SMTP client and starts to send all messages directed to the customer's [[domain name|domain]] (or domains) using SMTP protocol, via the same TCP connection. Since connections are initiated by customers, 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 [[Network Address Translation|NAT]]). Only the ODMR server, referred as the ''provider'', must listen to SMTP connections on a fixed IP address.

==External ODMR Support==
The design of ODMR allows the protocol to be used with an SMTP server that doesn't support it natively. A separate application can be used to do the initial ODMR protocol negotiation (<code>EHLO</code>, <code>AUTH</code> and <code>ATRN</code>), then hand over the connection to the SMTP server once the protocol has reached the "reversed" state.

==ODMR transport example==
A typical example of getting emails with ODMR. 

For illustration purposes here (not part of protocol), the protocol exchanges are prefixed for the server (>) and the client (<).
<syntaxhighlight lang="md">
>  220 EXAMPLE.NET on-demand mail relay server ready
<  EHLO example.org
>  250-EXAMPLE.NET
>  250-AUTH CRAM-MD5 EXTERNAL
>  250 ATRN
<  AUTH CRAM-MD5
>  334 <keysequenz>
>  235 now authenticated as example.org
<  ATRN example.org, example.com
>  250 OK now reversing the connection
<  220 example.org ready to receive email
>  EHLO EXAMPLE.NET
<  250-example.org
<  250 SIZE
>  MAIL FROM:<Lester.Tester@dot.foo.bar>
<  250 OK
>  RCPT TO:<l.eva.msg@example.com>
<  250 OK, recipient accepted
...
>  QUIT
<  221 example.org closing connection
</syntaxhighlight>

[[Category:Internet mail protocols]]