OK. This is my understanding of implement SPF.

On outgoing side, nothing to do with zimbra, just put a TXT record in your DNS of your domain. The following link from Microsoft could help you to...

See this from Microsoft web site: ( http://www.microsoft.com/mscorp/safety/technologies/senderid/default.mspx )

The Sender ID Framework is an e-mail authentication technology protocol that helps...

In Admin, "AS/AV" section, "Spam checking Settings":

"Kill percent:" and "Tag percent:" are mislead. I thought it's:

Kill 75 percent of all recognized spam emails

But, I believe,...