1. Not 100% if it works well since I've never personally used it, but from reading the information on it, it sounds like it can help against spam and what not.
2. from
wikipedia - "DomainKeys is an e-mail authentication system designed to verify the DNS domain of an e-mail sender and the message integrity."
3. SPF means Sender Policy Framework and from
wikipedia is "allows software to identify and reject forged addresses in the SMTP MAIL FROM (Return-Path), a typical nuisance in e-mail spam."
You can enable both DomainKeys and SPF under the Email Authentication page.
At the moment there are no tutorials here on Surmunity, but if you give me an hour or two I can whip some up for this? =)