SPF allows owners of an Internet domain to specifiy which servers are authorised to send mail for that specific domain. Authorised machines are listed in a DNS record that can be queried by receiving mail servers.

When a message is rejected due to SPF policy it means that you are attempting to send a message via a server that is not authorised in your domain's SPF record. This can occur if you have configured you mail client to send mail via your Internet connectivity provider's mail server.

It is recommended that you send mail through the mail server on your hosting account to prevent any problems.

More information is available on the SPF Website