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'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'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'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.
nosendetrn channel keywords
control whether the PMDF SMTP client sends an ETRN command at the
beginning of an SMTP connection. The default is
nosendetrn, meaning that PMDF will not send an ETRN
sendetrn keyword tells PMDF to send an ETRN
command, if the remote SMTP server says it supports ETRN. The
sendetrn keyword should be followed by the name of the
system requesting that its messages receive a delivery attempt.