When you send or view an email, most people do not realise that there are actually two versions of each mail. There (should be) a text version, and a HTML version. Some bad email programs (or programmers) do not generate both, which can cause major problems with viewing and deliverability.
While HTML in email is now common place, and most advertising emails are send as HTML, there are still some majpr challenges to creating HTML which will work across most major email clients.
One of the biggest differences is with Background Images and the main Body Tag of the email. While it is possible to add background images, actually making them show across the whole range is a serious challenge. For this reason, it is always best to design emails with the understanding that they will not always be shown as you intended.
As of 2016 the situation has not really improved. All style needs to be in-line, as the head-section is often stripped, and background-images are less-likely to be shown than they were in the past.
With all of this in mind, it is essential to keep email simple, and to throughly test the result in a miriad of situations. To this end, we maintain email accounts with lots of major suppliers so that we can test visibility for your email.