Here's a cool site which helps you test your MX records to first of all find out what they are, and then test diagnostics such as if your mail server is an open relay - very bad - on blacklists - also not good, or not performing well.
http://www.mxtoolbox.com/
For instance this site shows that intermedia.net has an smtp server that is potentially an open relay and responding very slowly.
Two electricmail.com servers return slowly and the second one has a reverse dns problem.
Check your mail servers by putting in your domain name, then run the diagnostic and blacklist test and ask your mail providers to fix any problems they find.