SPF records are used during email verification to prevent your domain name from being used by spammers or malicious users. Simply creating an SPF record on your main domain with the content: v=spf1 ip4:184.108.40.206 a -all (replacing 220.127.116.11 with your mail server’s IP address) will tell email receivers that your mail server is the only server allowed to send emails from your domain, all emails received from other servers are to be rejected or marked as spam. If you have multiple mail servers, you can add another ip4:x.x.x.x after the previous one to allow another IP address.
Note: The SPF record is still supported but shouldn’t be used in new configurations. Instead, making a TXT record with the same content is the new standard. Commonly, mail servers will define both an SPF and a TXT record for the most compatibility.