ETRN

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ETRN (short for English Extended Turn ) is an extension of SMTP - email - protocol for mail services from networks that do not constantly via a dedicated line to the Internet are connected. It is defined in RFC 1985 .

With an existing dial-up connection, an ETRN-capable e-mail server is contacted for sending and receiving e-mail / s (queues) from a requesting e-mail server. (Connection flow between e-mail servers - not to be confused with the client-side connection establishment as with POP3 .)

The ETRN command is an extension of the TURN command in SMTP. The SMTP TURN command reverses an SMTP connection so that the server can deliver e-mails over an existing connection. Since the SMTP-TURN command is a major security problem, the ETRN command was introduced instead.

syntax

ETRN [<option character>]<node name><CR><LF>

The node name must be a full name of a domain , which corresponds to a CNAME resource record or an MX resource record in the DNS .

Possible responses from the server: 

250 OK, queuing for node <x> started
251 OK, no messages waiting for node <x>
252 OK, pending messages for node <x> started
253 OK, <n> pending messages for node <x> started
458 Unable to queue messages for node <x>
459 Node <x> not allowed: <reason>
500 Syntax Error
501 Syntax Error in Parameters

Examples

Server: 220 xyz.com ESMTP Server
Client: EHLO my.fqdn.com
Server: 250 xyz.com
Client: ETRN
Server: 501 Command not accepted

...
Client: ETRN myfqdn.com
Server: 253 OK, 3 pending messages for node myfqdn.com started

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