In cases where the network connection between two systems is only available at particular times---a "dial up" sort of connection for instance---there is an SMTP extension whereby one system can inform another that it is ready to receive mail. This is performed using the SMTP extension command ETRN, defined in RFC 1985:⁴ the side that desires to receive mail connects to the remote side's SMTP server and issues the command ETRN receivinghostname. If the remote side's SMTP server supports the ETRN command, it will then attempt delivery of any messages it has waiting to be sent to receivinghostname.

The PMDF SMTP server supports ETRN. In particular, the PMDF SMTP server interprets a received ETRN hostname command as a request to run the channel which hostname matches, a received ETRN @hostname as a request to deliver all messages in the hostname subnet, and a ETRN #channelname command as a request to run the channel channelname. By default, the PMDF SMTP always responds to a remote site's ETRN requests; if you want to restrict this behavior, see Section 2.3.4.33.

And outgoing PMDF SMTP-based channels, such as TCP/IP channels, can be configured to send an ETRN command at the beginning of an outgoing SMTP dialogue via the sendetrn channel keyword; see Section 2.3.4.32. For instance, suppose a system host1.example.com has a dial-up connection to a remote system intermittent.some.where.com, where the intermittent.some.where.com system also supports ETRN. For a channel for connecting up to the remote side and sending ETRN, such a site might use a channel definition along the lines of:

tcp_dialup smtp mx daemon intermittent.some.where.com \ periodic sendetrn host1.example.com TCP-DIALUP