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.
I have an SPF record in my DNS records but I checked my SPF through MxToolBox (Domain Health Check - Online Domain Tools - Blacklist, Email, Website, DNS - MxToolBox) and I received a message "SPF record not found".
Here is my SPF Record:
aisonesystems.com. TXT v=spf1 +a +mx...