DNS has always been an interest of mine. Reading all of O'Reillys DNS book and joining AUDA when it was founded in Australia. Still a lot of people just don't get it ...
I stumbled across a good DNS checker site http://www.dnsreport.com/. You type in your domain name and it does a full test of everything, especially the MX and mail servers. In the times of increasing SPAM the setup of your MX is becoming real important as many providers are becoming so pedantic about everything being just right.
Its because of MX problems that I found this site. My provider, godaddy.com have some nasty email requirements. Sometimes people get a "553 Bogus helo" errors when emailing people hosted at GoDaddy. Can be hard to get this fixed.