If you care about email deliverability, you should follow Al Iverson’s Spam Resource Blog. You’ll always find little nuggets of email wisdom over there. In this article, at the bottom, read about Hotmail’s "Silent Discarding" problem, and his recommendation for fixing it. SenderID and SPF are ways of authenticating your emails, so they don’t look so suspicious to ISPs and other receiving servers. For MailChimp customers, authentication is free and automatically activated for all your emails.