Forwarding emails causes softfail on Gmail

HI!

I setup an email account into my Plesk 12 to automatic forwarding all emails to an gmail address. But Gmail put all mails received into the Spam folder.

I revised the mail Header and its a SPF softfail causes because my server is not rewritting the sender envelope.

How can I activate SRS on my Plesk or solve this issue?

Thanks!