How do I test a domain for DNS and mail server issues?

This process is for testing email receipt to verify if mail problems are DNS or mail server related.

1. Go to http://mxtoolbox.com/SuperTool.aspx
2. Enter <domain.com> in the text box and click MX Lookup
3. Click Find Problems
4. Check for Errors under Mail Server and DNS
   1. If you see errors under Mail Server, report them to your mail provider
   2. If you see errors under DNS, report them to your DNS provider