The ETRN command can be used to release your domain queue on a remote backup server only if your server has a static IP address.
If your server has a dynamic IP address, the ETRN method does not work, since the backup server does not know the IP address your server is using, and the backup server is not able to open a connection to your server.
If your server uses a dynamic IP address, it should use the On-demand Mail Relaying method to retrieve mail from the backup server.
When On-demand Mail Relaying method is used, your server connects to the backup server, authenticates itself, and then it issues the ATRN command. Then the servers exchange their roles and the backup server starts to send your server your domain mail via the same channel. This eliminates a need for the backup server to open a connection to your server.
Since the backup server does not open a connection itself, it has to verify that the server that sends the ATRN command and wants to retrieve your domain mail is really your server. Your server should provide some name and password that should be accepted by the remote server and that should allow your server to issue the ATRN command.
Consult the remote server administrator to learn the name and the password your server should send before sending the ATRN command.
Use ATRN: Select this option to use the ATRN (On-demand Mail Relaying) method instead of the ETRN method.
AUTH Name and Password: This pair of strings is sent to the remote server using the AUTH command. The access rights granted to this login name on the remote server should allow your server to use the ATRN command.
The CommuniGate Pro SMTP module uses the AUTH CRAM-MD5 authentication method to send passwords in an encrypted form. If the remote server does not support the CRAM-MD5 method, the clear-text AUTH LOGIN method is used.
If your backup server does not support On-demand Mail Relaying, you should use the Unified Domain-Wide Account method implemented with the RPOP module.
The RFC2645 suggests to use the special TCP port number 366 to provide the ATRN services. If your backup mail server provides the ATRN services on that port (or on any port other that the standard SMTP port 25), you should specify the port number in the Send Wakeups To setting field. Use the colon symbol to separate the server name and the port number: mail.provider.dom:366
You can use secure communications with the backup server if you include the backup server name into the Send Encrypted list.
When the backup server name is specified, the SMTP Settings page displays the Wake Up Now button. Click that button to initiate a wakeup session immediately.