Many thanks again, RogueCoder. It now almost works. Besides adding your code in virtual file, I have to define virtual_alias_domains in main.cf, right? The problem is that I don't know the value to be used in virtual_alias_domains for "any domain". This is my code:
virtual_alias_domains = (.*) <--- The problem is in this line, I think- It is not working
virtual_alias_maps = hash:/etc/postfix/virtual
Otherwise, it should work. However, I just predict a further problem. If a sender uses "bcc:" and then the email is sent to admin, as configed. Can the admin see the email list in bcc field? How can the admin see it? Thanks.