1 Why SPF ?
Sender Policy Framework (SPF) is a simple email-validation system designed to detect email spoofing by providing a mechanism to allow sending mail exchangers to check that sending mail from a domain comes from a host authorized by that domain's administrators. The list of authorized sending hosts for a domain is published in the Domain Name System (DNS) records for that domain in the form of a specially formatted TXT record. It authenticates a sender’s identity by comparing the sending mail server’s IP address to the list of authorized sending IP addresses published by the sender in the DNS record. Email spam and phishing often use forged "from" addresses, so publishing and checking SPF records can be considered anti-spam techniques.So, based on your issue you are in the need of adding SPF records in your domain.
2 Steps to be followed to Generate SPF record :