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: ⁶ 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 domainname command as a request to deliver all messages to domainname , a received ETRN @domainname as a request to deliver all messages in the domainname 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 2.3.4.34.

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 2.3.4.33. 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