Exim: How to turn off DKIM for forwarded mail?

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http://serverfault.com – I have DKIM configured in Exim for outgoing mail, as per the documentation. Exim signs all outgoing mail. But some of that outgoing mail is forwarded, thanks to a users .forward file. This is a problem for me, because some of those messages are spam (my exim configuration does not do any verification) and I don't want to take responsibility for them. But I can't figure out how to configure Exim not to sign these messages. My configuration is basically the Debian Squeeze default, with a few DKIM_* macros set. I can post more details, but I think seeing any example of conditional DKIM sign (HowTos)