The allowetrn, blocketrn, domainetrn, and silentetrn keywords control the MTA response when a sending SMTP client issues the ETRN command, requesting that the MTA attempt to deliver messages in the MTA queues.
By default, the MTA will attempt to honor all ETRN commands; that is, the allowetrn keyword is enabled. You can specify that the MTA not honor ETRN commands by including the blocketrn keyword in the channel definition.
You can specify that the MTA honor all ETRN commands, but without echoing the name of the channel that the domain matched and that the MTA will be attempting to run by including the silentetrn keyword. The domainetrn keyword specifies that the MTA honor only ETRN commands that specify a domain; it also causes the MTA not to echo back the name of the channel that the domain matched and that the MTA will be attempting to run.
disableetrn disables support for the ETRN command entirely; ETRN is not advertised by the SMTP server as a supported command.