Using Classic Outlook on Windows for personal email, calendar, and contact management
I mentioned that on page 4:
Basically the problem is this, major mail providers (Gmail, Yahoo etc) start enforcing new policy. If your email domain does not conform with their new policy, they are not going to accept your email until you fix it.
If you post your domain name (do NOT post your entire email address), we may be able to tell you more about the problem.