Sendetrn, nosendetrn Channel Options

Sending an SMTP ETRN command
The extended SMTP command ETRN (RFC 1985) allows an SMTP client to request that a remote SMTP server start up processing of the remote side&#x27;s message queues destined for sending to the original SMTP client; that is, it allows an SMTP client and SMTP server to negotiate "switching roles", where the side originally the sender becomes the receiver, and the side originally the receiver becomes the sender. Or in other words, ETRN provides a way to implement "polling" of remote SMTP systems for messages incoming to one&#x27;s own system. This can be useful for systems that only have transient connections between each other, for instance, over dial up lines. When the connection is brought up and one side sends to the other, via the ETRN command the SMTP client can also tell the remote side that it should now try to deliver any messages that need to travel in the reverse direction.

The SMTP client specifies on the SMTP ETRN command line the name of the system to which to send messages (generally the SMTP client system&#x27;s own name). If the remote SMTP server supports the ETRN command, it will trigger execution of a separate process to connect back to the named system and send any messages awaiting delivery for that named system.

The  and   channel options control whether the  SMTP client  sends an ETRN command at the beginning of an SMTP connection. The default is, meaning that the MTA will not send an ETRN command. The  channel option tells the MTA to send an ETRN command, if the remote SMTP server says it supports ETRN. The  requires an argument giving the name of the system requesting that its messages receive a delivery attempt to send in the ETRN command.

See also:
 * TCPIP channels
 * SMTP and LMTP protocol channel options
 * Channel options