When setting up Microsoft 365 and using MS Outlook for you mail client, you will need access to your domain registrar and DNS records. You need to login to your domain name registrar access the DNS management cPanel and add an new TXT record for the SPF if none is found, otherwise update the current SPF record.
If you need to contact Microsoft support 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
- We recommend using ~all when setting the SPF record instead of -all.
Check SPF Settings: https://mxtoolbox.com/spf.aspx