I would like to have a separate queue per domain for hosts which dial in to collect their mail.

Exim isn't really designed for this kind of operation. The only way to do this would be to cause it to send those messages to a differently configured version of Exim with its own spool area. This could be done via a pipe or SMTP to a private port. The main Exim, listening on port 25, would then be configured to run an appropriate command to prod one of the others when it received ETRN, by means of the smtp_etrn_command option. You could probably manage this with a single Exim binary and a number of different configuration files, passed to the special versions using the -C option. For this application they could all run as exim, since no root privilege would be needed. An alternative approach id to get Exim to deliver mail for such hosts in batch SMTP format into some directory, and have the ETRN run something to pass such messages to the dialled-in host. See also `../Q1403`_.