SPF know as sender policy framework.Its a domain name service which identifies mail server permission to send mail.The purpose of spf record is to prevent spammers from sending message.

Steps to add spf record:

1. Go to DNS
2. Go to forward lookup zone
3. select your zone
4. Right click your zone name
5. Create New Records
6. Scroll down choose Text (TXT)
7. Create Record
8. put field blank that says Record name
9. copy and paste the spf record into it.

http://www.stradsolutions.com/windows-vps.php