And previously (in response to the postconf -n result)
I didn't keep a record of each edit of the main.cf file. I did try things like adding emailistA.com to the mydestination list (and localhost.emailistA.com like for the maindomain.com), but nothing seemed to change.
There was also a value for milter_default_action which has been commented out as apparently it was added by opendkim.
Have you removed the alias domain from the emaillistA.com domain and similarly for the others and then regenerated the maps with mailman aliases or mailman start -g after removing the alias domains.
I've done that now and but it doesn't seem to have changed anything. At present emails to the emaillistA.com domain (vs the maindomain.com) are being shunted again.
Also, I'm confused about "User unknown in virtual alias table" since there are no virtual_alias_domains in your Postfix main.cf. Have you reloaded Postfix since updating main.cf?
I did again and this bounce message has disappeared - but instead it is shunting - so no bounce comes back now.
I'll go through the documentation again line by line and see if there is something I missed.