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

you have added mydestination = mydomain.com and so the valid users for mydomain.com 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.com for now
# Ensure that postmap -q mydomain.com mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf is returning mydomain.com 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

Open /etc/postfix/main.cf, look for the mydestination field and change it's value to the below

mydestination = localhost, localhost.localdomain

Don't forget to run sudo postfix reload afterwards