We just had a customer create her email content in Microsoft Word. Then, she pasted from Word into her HTML email, and sent a test to herself. That’s normally when most people learn how bad Microsoft Word is for making emails (it adds a whole bunch of code that breaks your email design). But this user never caught that, because she checked her test email in Microsoft’s Outlook 2007 (which uses Microsoft Word to render HTML email). So to her, everything looked just fine. So she sent the broken email to her entire list. Ouch.
In our experience, Microsoft Outlook has always been a very "forgiving" email app. Almost anything will work in Outlook, including videos in some cases. So if you want to do some serious email campaign testing, download a few other email programs. Mozilla Thunderbird is a good start. Of course, if you don’t have the time or desire to download a bunch of email programs and sending lots of test campaigns to each one, you can always use our built-in Inbox Inspector.