ETRN (Extended Turn) is an extension to the SMTP mail delivery protocol. When SMTP is used to collect email from the Internet, your Internet Service Provider (ISP) is responsible for connecting to your MultiNet server when mail is to be delivered. As most ISPs are not able to dial in to your network, they are only able to connect to your MultiNet server when your Internet Connection is active.
In order to determine whether your Internet Connection is active your ISP must either:
- continuously poll your MultiNet Server to gauge whether it can make a connection
- implement some form of trigger that enables MultiNet to inform the SMTP server that MultiNet is able to receive Internet email
The first option can result in significant computing overheads for your ISP. Your connection is unlikely to be open for much of the day and so most attempts to poll the MultiNet server will fail.
The original SMTP protocol offered a method for triggering the delivery of email called the TURN command. However, there are a number of security weaknesses in this method and it is not implemented by the MultiNet server (nor many ISP mail servers)
ETRN overcomes the security weakness of the original TURN command and works as follows:
When MultiNet connects to the Internet it contacts your ISPs SMTP server and issues a command informing it that your domain is ready to receive email. Your ISPs mail server responds by making a connection to the MultiNet server and delivers email for that domain.
The security enhancement in this approach is that your ISP does not need to 'trust' that the system connecting to it and requesting email to be dispatched is the correct system to deliver email to. The SMTP server uses its own knowledge of the owner of the domain to decide which system to connect to.
Note that some ISPs do not support ETRN. However, if it is supported then you may experience reduced email collection times by installing this service.
Note that ETRN is only beneficial to you if you use SMTP to collect Internet email. If you use POP3 to collect email then there is no advantage to installing this service.
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ETRN