Any web designer knows to test their websites against different browsers.  Email designing is no execption, except there are more browsers to test.  Theres: 4 different versions of Outlook, Entourage, MacMail, Outlook Express, and Thunderbird.  Then there’s gmail, hotmail, and yahoo.  Basically you can narrrow websites down to 3-4 browsers, but mail clients have a more even distribution.  Not to mention that there are a lot of methods of distribution including Campaign Monitor, MyEmma to name a few.  That was fun… but worth it because the outcome is the 3Cs – clean, clear, and corporate.