[exim] Strange problem with domainkeys

Hi, list!

I have a very strange problem with domainkey...
A friend of mine, tries to send an E-Mail from yahoo to an address that
forward this E-Mail to my addresse.
And my Exim refuses the E-Mail saying that the signature is invalid...

Has someone an idea why it does?

I think that domainkeys offer a solution to the forward... If they don't, I
can deactivate them... :)

Thanks
Luca Bertoncello
(lucabert@???)