SPF is the Sender Policy Framework, a way to use DNS to tell the world about the servers that can legitimately send email for your domain. It is an anti-spam tool that makes it a little easier to identify spam, and a little harder for spammers to hide.
We've noticed that many of our members register more domains than they need for email purposes. For example, someone whose domain is example.com may also register example.net or other similar domain names to prevent competitors from creating similarly-named sites. But they only use their primary domain for email.
Our "SPF Email Protection" feature is an easy, automatic SPF record that will tell the world that your domain isn't used for email. That will help foil spammers who might otherwise forge email from that domain, as described in this FAQ entry. That means that spammers have one less place to hide, and also that you won't get their bounce messages.
Our standard offering is designed to be easy to use for this common case. If you need more specific settings for SPF, you can create your own custom TXT record for your domain containing any SPF code you want. The specific code you need depends on the server you use to send (not receive) email, which is usually your ISP's mail server.
To find out what SPF code you should use, or to learn more about SPF and how it can help you, check out the SPF home page.
SPF isn't suitable for all cases. For example, it doesn't work well if your domain sends email from a number of different or frequently changing places. But where it is suitable, it both helps you a little bit, and it's a way to be a "good netizen" to your Internet neighbors by making spamming just a little bit harder.
Anyone who wants to send mail from their domain from our servers and wants an SPF record for it should refer to this FAQ entry instead.