SPF (Sender Policy Framework) is a mechanism used to reduce email forgery, allowing you to declare where on the internet your domain sends email from.

Libra Esva supports it as integrated check in it's multi-layered antispam engine.

The general idea is that a recipient email server can check your SPF records and if the sending email server is listed in these records the email will be accepted. If the sending server is not in the list then the email can be further actioned upon, such as rejected or held in quarantine.

One of the benefits of declaring an SPF record is the reduction of backscatter spam.

It is important that email administrators manage SPF correctly if they are using it. If a sending email server is being moved to a new hostname or IP address which hasn't been declared in the SPF records, then any recipient email servers which use SPF as part of their mail filtering may block emails coming from the domain and sent from the new server unless they are whitelisted.

Additionally, if users change the outbound server setting (for example to their ISP's outbound email server) on their email applications and their domain uses SPF, they may not be allowed to send to other users whose mail server checks SPF records as part of their filtering.

The SPF records for a domain are defined in the domain's DNS records.

Further information on setting up SPF records for your domain can be found here: http://old.openspf.org/wizard.html

General SPF information: http://www.openspf.org