Adding custom header

[Jack Knowlton]

Hi guys.
My mailserver uses a relayhost which basically only signs messages with
DKIM and delivers them.
I would like to add a custom header, something like "X-Abuse: if spam/uce
email ab...@mydomain.com".
How should I go about, without postfix adding the header twice (before and
after dkimfilter-out)?

Thanks.

-JK