insert a X-header without resending the mail

Hallo

i want to insert a X-CHECK: header field in emails i receive.
when i use 
 
:0 
* !X-Check: IS
|formail -A "X-Check: IS" |/usr/sbin/sendmail -t

there is a new "Received: from" in the Received Email because 
SENDMAIL resends the Email.

is there a way without resending the email with formail
to add a new header field ?

maybe with SED ?

so far i understood sed  need a exsiting field to change or add something like

| sed -e 's/^Subject: \(.*\)/Subject: \1 \[#Filter Namibia Mails#\]/'

???

Matthias






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