Check your dovecot sql configurations. If the users and aliases are in MySQL, Dovecot needs to be able to figure out who or what they are. It can't do that unless it is able to get that information from MySQL.

The problem, as indicated, is that Dovecot has no idea who the alias is because it is not actually an email account. Postfix should be telling it which account(s) to deliver it to. The problem is more than likely how you have configured Dovecot in your master.cf file.