How to add a header indicating recipient domain?

I have set up a mail gateway using Ubuntu 8.04/postfix/clamav/
spamassassin/mailscanner. It stands in front of a single MS Exchange
2003 R2 server serving four mail domains. Mail to username@whichever-
domain.com is delivered to the Exchange inbox for username, which is
what we want. However, we need to flag mail addressed to one domain in
particular for special handling (say special.com).

Surprisingly, neither Exchange nor Outlook can reliably identify the
recipient domain as a rule condition. (They can check for text in
headers, though.) Exchange does have the hooks for coding this, but
writing, testing and maintaining an SMTP sink would be a major
undertaking for such a seemingly minor result.

I feel sure that postfix or mailscanner could handle this easily,
i.e. if the recipient domain is special.com, add a header "X-
Special:true". Am I right, and if so, can anyone point me to the right
place to configure it?


TIA