Using Classic Outlook on Windows for personal email, calendar, and contact management
If you use custom domain, the solution is to create all required DNS records for your domain name.
Another solution is to do nothing about it and accept the fact that your email domain name will no longer be able to send email to many major mail providers. Not trying to be rude. Just telling you the facts. Until you create those (SPF, DMARC, DKIM) records, the problem will never be resolved.
Actually the fact that you quoted MSFT employee who gave you a solution but completely ignored the solution tells us that you don't understand it at all. What you need to do is hire an email administrator for your business or talk to them if you have an IT department.