Sieve and forward

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