ATRN is a new SMTP mechanism (RFC 2645) that allows you to temporarily hold mail for a remote mail server. Also known as ODMR (On Demand Mail Relay) this process makes it possible for the servers with unreliable Internet connections to have their own mail server without having a static IP address.

Unlike ETRN mechanism, the destination mail server is not published in the DNS so there is no need to create A or MX records. Furthermore, ATRN is more secure than ETRN because it is an authentication mechanism (Authenticated TURN as the name implies) that requires the remote SMTP server to provide authentication before the mail is sent. With ETRN a rogue SMTP server could hijack the static IP address and steal your email, a problem that the authentication layer solves. ATRN is also more convenient for planned downtime and maintenance intervals where you do not want mail arriving to your server just because of a higher MX priority required by ETRN.

The only bad news is that the Microsoft Exchange 2003 server is not fully RFC compliant, but as long as your setup is not too exotic it will work just fine. The only known issue applies when you try to authenticate with one server to download mail and another server to send (relay) mail.

This document will help you create the server component of the ATRN mechanism that will hold the email for the remote, and unreliably connected, SMTP server.

Creating an SMTP Connector

First start your Exchange System Manager, usually by clicking Start > All Programs > Microsoft Exchange > System Manager. Open your Connectors folder, right click and select New > SMTP Connector.

General Setup

First give your SMTP Connector a friendly name. We are setting up an ATRN server so I called it RealMCP ATRN Server.

Because we are holding the mail for the remote domain that does not have a static IP address, we will select “Forward all mail through this connector to the following smarthosts” and enter the hostname of the SMTP server that will be downloading the mail from you.

Next, add your ATRN server as the local bridgehead for the SMTP connector. Click on Add and select the mail server you wish to use as the ATRN server.

Domain Relaying

Because ATRN functions on per-domain basis you need to define the domain names that you will be accepting mail for and relaying it to the ATRN client (remote server.) Select the Address Space tab, click on Add and type in the domain name that you wish to queue mail for. Check the box “Allow messages to be relayed to these domains” to enable your ATRN server to accept mail for the remote domain. For demonstrational purposes I’ve selected sbsguide.com as the domain this ATRN server will be queuing mail for.

Setup ATRN Delivery Option

Now that you are accepting mail for the remote server, you need to specify how that ATRN client will download mail from you. Because ATRN requires authentication and is an on-demand mechanism, you have to specify a username that can trigger remote delivery.

Select Delivery Options tab and select “Queue mail for remote triggered delivery.” Click on the Add button to add a user that will have permissions to trigger delivery and click on OK. Click on OK to create your SMTP connector and you’re done.

Final Steps

To apply settings described above you have to restart the Microsoft Exchange Routing Engine and the Simple Mail Transfer Protocol (SMTP) services.

Additionally, you have to point the MX record of the domain you are relaying (sbsguide.com) to your ATRN server. When the mail is sent to that domain, it will be delivered to your ATRN server and wait until the remote server (ATRN client) triggers delivery.

Read my other Exchange articles:

Publishing SenderID records for Exchange SP2 IMFv2
Enabling IMF 2 in Exchange 2003 SP2
Changing Exchange 2003 Store Database Limits
Exchange 2003 SP2 for SBS
Modifying the Outlook Web Access Login Page
Disabling NDR (non-delivery reports) on Exchange 2003
Setting up Exchange 2003 as an ATRN Client
Setting up Exchange 2003 as an ATRN Server