Sieve and forward

[Il Neofita]

Hi
I have already posted to the postfix group, however, I believe that sieve
and dovecot should be able to fix this problem.
When I receive a message from yahoo and the user forward it to a gmail or
yahoo acount this email is considered as spam or rejected.
>From yahoo is rejected since it seems that I am try to send spam since the
email should be signed with dkim.
Is there a way to encpuslated or sign in some way.

Thank you