[otrs] use "X-Envelope-To" header insted of "to" header

Hi, our MS Exchange server thinks it's funny to rewrite all "to" headers to the mailbox's default mail address. However, I want to see the orinigal target of a mail in OTRS. The original "to" is transported in a header named "X-Envelope-To". I fetch my mail via IMAP oder POP3. How can I configure OTRS to use the "X-Envelope-To" instead of the "to" header? Thx. Timo Rupp