I have an SPF record in my DNS records but I checked my SPF through MxToolBox (https://mxtoolbox.com/emailhealth/aisonesystems.com/) and I received a message "SPF record not found".
Here is my SPF Record:
aisonesystems.com. TXT v=spf1 +a +mx +(server-IP).plesk.page -all
I was adding my SPF record in Plesk DNS records. I bought my domain from Contabo. So I add the SPF record in Contabo Server and the issue was resolved.