What is SPF?
SPF, or sender policy framework, is a means of preventing sender address forgery. The SPF system allows you to specify servers and IP addresses that are authorized to send mail from your domain(s). This feature works to prevent outgoing spam messages.
How to Enable SPF
Enable SPF in the control panel. It is under Email > Authentication. Under SPF, click on Enable. The status should show SPF is enabled and DNS check has passed.
Note that SPF is for a domain.
How to Verify SPF
You can verify that your SPF record is valid by sending an email to a gmail account. Open the gmail, click on the arrow under Reply and choose "show original". You will find a line that has spf=pass at the beginning.
It is not necessary to use SMTP; PHP's mail works fine.