You need to login to your Domain registrar or DNS management cPanel and update the current SPF TXT record or add an new TXT record for the SPF if none is found on your Domain's DNS.
Follow the instructions from SPF TXT record for Office 365.
More information can be found https://docs.microsoft.com/en-us/office365/securitycompliance/how-office-365-uses-spf-to-prevent-spoofing#SPFSyntaxO365.
If you need to contact Microsoft click here for local contact info.
-all means only this IP is authorized to send mail for the domain.
~all,softfail,Allow mail whether or not it matches the parameters in the record
+all,pass,Allow all mail
Check SPF Settings: https://mxtoolbox.com/spf.aspx