Overview
The Sender Policy Framework (SPF) is an open standard specifying a technical method to prevent sender address forgery. More precisely, the current version of SPF — called SPFv1 or SPF Classic — protects the envelope sender address, which is used for the delivery of messages.

Like paper mail letters, e-mail messages have at least two kinds of sender addresses: one on the envelope and one in the letterhead:

• The envelope sender address (sometimes also called the return-path) is used during the transport of the message from mail server to mail server, e.g. to return the message to the sender in the case of a delivery failure. It is usually not displayed to the user by mail programs.
• The header sender address of an e-mail message is contained in the "From" or "Sender" header and is what is displayed to the user by mail programs. Generally, mail servers do not care about the header sender address when delivering a message.

(There are other solutions that protect the header sender address or that do not care at all about who sent the message, only who originally wrote it.)

Even more precisely, SPFv1 allows the owner of a domain to specify their mail sending policy, e.g. which mail servers they use to send mail from their domain. The technology requires two sides to play together: (1) the domain owner publishes this information in an SPF record in the domain's DNS zone, and when someone else's mail server receives a message claiming to come from that domain, then (2) the receiving server can check whether the message complies with the domain's stated policy. If, e.g., the message comes from an unknown server, it can be considered a fake.

For more information, please visit the official SPF website: http://www.openspf.org/

Updating your SPF record for use with our "Standard SMTP Relay Service" plans:
If your company has defined an SPF record and you are using our Standard SMTP Relay Service, you should update your SPF record to include our servers: 

Example:

YourDomain.com. IN TXT "v=spf1 include:CHillox.net ?all"

For more information, please review the SPF Record Syntax: http://www.openspf.org/SPF_Record_Syntax#include

SPF Wizard
For additional assistance in setting up an SPF record, visit http://www.openspf.org/SPF_Record_Syntax