ETRN (Extended TURN) is SMTP protocol extension for on-demand mail relay with DNS resolution with fixed IP address. It can be used to release the load of the destination server by using a gateway to filter spam and viruses.

ATRN (Authenticated TURN) can be used when the destination server does not have a fixed IP address and/or is not permanently connected to Internet. It is more secure and recommended to be used.

The session is initiated by the destination server, which then issues ETRN or, after authentication, ATRN command. Both, "backup" server and client server need to be set up to use ETRN/ATRN.

The procedure of execution of this command is: destination server (client in this session, let's call it A) connects to the one (server in this session, let's call it B) that holds mail for it, issues ETRN or ATRN command, they exchange their roles so B becomes client and A becomes server. What follows is normal SMTP session with MAIL FROM commands etc. That means, all filters and antivirus are applied by destination server.

Backup Server Setup

The first step on the destination server setup is to create a domain and set its type to ETRN/ATRN queue domain in the combo box in the domain's properties on the Accounts tab. If ETRN will be used (destination server has fixed IP address), Value: field should contain this IP address. If the server has dynamic IP, leave this field empty as in the picture.