Triggering message transfer with remote SMTP systems

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: the side  that desires to receive mail connects to the remote side&#x27;s SMTP server  and issues the command     . If the remote side&#x27;s SMTP server supports the ETRN command, it will then  attempt delivery of any messages it has waiting to be sent to   .

The MTA&#x27;s SMTP server supports ETRN. In particular, the SMTP server interprets a received      command as a request to run the channel which    matches, a received      as a request to deliver all messages in the    subnet, and a      command as a request to run the channel   . By default, the SMTP server always responds to remote side&#x27;s ETRN requests; if you wish to restrict this  behavior, see   and related channel options, or the    mapping table.

And outgoing 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    channel option. For instance, suppose a system host1.acme.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.domain.com TCP-DIALUP As of iMS V5.0, there is an   mapping table. Probes have the form: transport-info&#x7c;app-info&#x7c;channel-to-run&#x7c;full-name&#x7c;claimed-system (Here   is the ETRN parameter, and   is a  processed version of that parameter.) If the mapping table returns a  ,  ,  , or  ,  then the ETRN command is rejected with a   error; by  default 459 4.5.0 Cannot start delivery on channel - access denied or if alternate text is included in the  entry,  then the alternate text is used.

If the  mapping table returns a    or , then the ETRN is  attempted. If the mapping table also returns a   or    , then the MTA tries to lookup   (in the channel/host table from the configuration file) and if that lookup is successful, runs that channel (rather than whatever channel the original ETRN command might have run).

See also:
 * allowetrn Option
 * disableetrn Option
 * sendetrn Option
 * ETRN_ACCESS mapping table
 * Routing via gateway systems
 * TCPIP channels