[exim] Removing header from mail in spool?

Hi,

I am finally working on changing my config to wo 4.94. There is one
thing I could not get working so far and this led to mails being in
limbo on my secondary mx. I would need to delete a certain header once
from those mails because now it is twice in the mails. When it is only
once in there my primary mail-servers should be able to accept those
mails again.
Is there any way to remove a header from those mails in limbo?

Best,

Niels