ETRN, short for Extended Turn, is an extension to the SMTP mail delivery protocol. ETRN is a command that starts the delivery of queued mail for a specific domain.
Mail sent to an ETRN domain is simply accumulated and not sent anywhere. When SMTPMax is contacted and the ETRN command is sent to it by the client, the mail is "de-spooled" to the correct mail host.
Example:
All email coming in for TESTCOMPANY.COM are queued in the ISP/Exchange mail server. The TESTCOMPANY.COM in-house mail server issues the ETRN command at scheduled intervals to the ISP/Exchange mail server. When the ISP/Exchange server receives the ETRN command, it looks at the domain name that is specified in the command and makes a local decision based on the IP address from which the request has originated.
Now the ISP/Exchange server starts de-spooling TESTCOMPANY.COM’s messages to the remote site that is TESTCOMPANY.COM’s mail server. The in-house mail server distributes the messages to appropriate local mailboxes.
What is a benefit of using ETRN?
This cost effective ETRN mail spooling service enables small/mid sized companies to reduce their cost by not having a dedicated internet connection.
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TESTCOMPANY.COM ---- sends ETRN request ----> ISP/Exchange Server
ISP/Exchange Server ---- TESTCOMPANY.com mails are de-spooled to ----> TESTCOMPANY.COM
TESTCOMPANY.COM:
The client Host that issues the ETRN command at scheduled intervals for mail de-spooling.
ISP/Exchange Server:
The ISP/Exchange Server queues the mails for client domain (ex. TESTCOMPANY.COM)
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Here is another more specific example:
24.248.49.25 -
:: This server does NOT have a dedicated connection to the internet.
:: This server has a domain called test.com that is setup and ready to send and recieve email.
24.248.49.26 -
:: This server DOES have a dedicated connection to the internet.
:: This server has a domain called localtest.com setup that can send and recieve mail.
The users of test.com (24.248.49.25) need to send email to localtest.com (24.248.49.26), but since they do not have a dedicated internet connection they have to wait until 2:00 PM when they dialup to their local provider for a connection. They are only connected to the internet for 10 minutes. Obviously, people need to compose and get email ready at other times of the day than in that 10 minute window.
Server 24.248.49.25 needs to setup a domain in MailMax called localtest.com and set it up as an ETRN account and the delivery address needs to be 24.248.49.26. During the day users of test.com can compose messages to accounts on localtest.com and send them as normal. All the email is accumulated in the local localtest.com account. Now, these messages just need to be sent when .25 has a connection to the internet. Once they do have a connection, the Administrator has to telnet to ther local IP address. Then they need to give the ETRN command. In this case it is, ETRN 24.248.49.26
All of the accumulated email from test.com to localtest.com is sent to .26.
