When you design your email campaigns make sure you test them in various email clients. HTML, CSS, and other code/design can display differently in Gmail, Yahoo, MSN, Outlook, and Thunderbird. In addition to testing different clients you may also consider different browsers. While most core code is universal among browsers, good programers and designers know that you have to test everything to make sure it renders correctly in each of the most popular browsers.
Recently, Litmus put together a compelling infographic about the various popularity of different web browsers when specifically using webmail services. We found it to be very informative. With their permission we are including it below.
Click on the graphic for an enlarged view.