can you login to mysql using the user/pass you setup for that mysql database?
maybe its a communication problem with mysql. If postfix cant login to check the usernames. are you sure the user in the DB is EXACTY correct? could you try recreating it and see if that helps?
I can tell by the logs that postfix is receiving the mail correctly, passing it off to amavis correctly, its coming back from amavis correctly (250 OK) but its getting hungup on the user lookup part. which is why i say to check communication from postfix to mysql aka the user/pass of the user you setup for the mail DB, aka mail_admin or something similar.