SPF provides a way for a domain to tell the world that mail "from" that domain should only arrive from a specific list of mail servers. If the recipient's mail server checks, it can reject mail that arrives "from" a domain that comes from any other mail server.
The default SPF record sytanx :
v=spf1 -all -> it means mydomain.com never sends mail. We may need to do this when a domain is only used for web services and doesn't do email.
v=spf1 mx -all -> If the MX servers used to send mail.
v=spf1 a mx -all -> if the web server and the mx servers used to send mail.
You can enable SPF records by add txt record from your DNS manager, put the SPF record syntax as the txt record value.
for the complete syntax reference : http://www.openspf.org/SPF_Record_Syntax
