case sensitive system user names & postfix

[Vizion]

Freebsd 6.1 running Postfix 2.3.2

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Here is the problem:
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I only need this for sensitive unix accounts on the mail server. I have found the following method cuts spam enormously to such accounts BUT I having just moved to using Postfix dor the mailserver I do not know how to condigure postfix so that it works.

Where for example the administrator may have the case sensitive user account of "AdMiniStraTor". Hence only mail from correspondents who use a correct case formating in the TO address are the only senders whose mails are intended to get through to those unix accounts on the local machine. 

When postfix receives a mail correctly addressed to AdMiniStraTor@$myhostname it tries to look up the user "administrator" and reports there is no such user and rejects the mail as user not found.

Other mail mail accounts are addressed to user@$mydomain and forwarded by carious methods and are not case sensitive.

How can I configure Postfix to deal with this problem?

Postfix is running on a separate host behind a firewall with all mail port requests being past through the firewall.

Thanks

david