SPF creation Help

I have got Zimbra Collaboration Suite 5.0 Setup, 

i am receiving a lot of Spams, and Backscatter Spams as well.

Actually, i need please dedicated Practical steps to follow to create a successful SPF record creation to prevent
the Spams and specially the Backscatter Spams on Zimbra Server.

And does this SPF record has to be applied on both our own Zimbra mail server and the Mail Server's Public IP at
ISP to prevent Spams and Bacscatter Spams.

Your replies will be highly appreciated.

Thanks a lot.

Torinto
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