I haven't resolved this yet.
In dovecot-deliver.log I've found this:
deliver(someuser@mydomain): Info: msgid=<7dc18b1996e8f9250ebcce3d2859a9f3.squirrel@1 18.104.22.168>: saved mail to INBOX
someuser@mydomain does not exist and folder someuser is created in /home/vmail/mydomain.
But when I remove allow_all_users=yes in userdb static section in devecot.conf, mail is not "delivered" (new folder not created) but no mail message to sender that delivery failed.
Should postfix check if user exists before passing to dovecot for delivery?