If you're sending emails to firstname.lastname@example.org
you have to make sure that mydomain.tld is in mydestination
. Currently it isn't!
Then you must have a system user yyy
If yyy is just an alias, not a system user, you have to run:
postconf -e 'virtual_maps = hash:/etc/postfix/virtusertable'
Then create /etc/postfix/virtusertable with the following contents:
Whenever you change /etc/postfix/virtusertable, you have to run
Don't forget to restart Postfix after your changes.