Need change "To" field when send mail to a list

[dalmate]

I have a question:
When I use zimbra to send mail to a list. In the recipient,he sees the "To" field is list's address (it's too bad because he'll know it). We want to rewrite "To" field to members of list. Such as:
-we send mail to list@test.com, its members are a@test.com and b@test.com
and in a's mailbox, he'll see "To" field is: a@test.com and b@test.com

[dalmate]

could you please give me an addvise, whether the following is possible with
postfix? I'd like to manage distribution lists with postfix, so that when a
user sends a message to

listX@mydomain

postfix looks up an alias map in MySQL and sends individual emails to each
alias found in the table. It is important that the To: header of the mail
contains the expanded address, not the original one so that the resulting
mails looks exactly as if the user would have send them individually to each
recipients