Oh, my mistake. i just realized you need sender dependent relay setting, not per-domain relay.
*) Per-domain relay you set with iRedAdmin-Pro is used when you RECEIVED an email, then Postfix will relay this RECEIVED email to specified relay host.
*) You need sender dependent relay setting. Please refer to Postfix official doc for more details:
http://www.postfix.org/postconf.5.html# … yhost_maps