Postfix - status=bounced (unknown user "myuser")

you have added mydestination = mydomain.example and so the valid users for mydomain.example will be searched in local_recipient_maps. It is very clear from the logline that local delivery agent is selected to deliver the mail, as local domain class matched. See postfix address classes for more help

Modify your main.cf like below

#/etc/postfix/main.cf
mydestination =
# set to localhost localhost.$mydomain or remove mydomain.example for now
# Ensure that postmap -q mydomain.example mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf is returning mydomain.example and
# postmap -q [email protected] mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf succeeds

Use following settings in /var/postfix/main.cf

myorigin = localhost
myhostname = <your_host_name>
mydestination = localhost.$mydomain, localhost, localhost.localdomain

followed by:

sudo postmap /etc/postfix/virtual
sudo service postfix restart