This help article is for an old version of Direct Mail.
A design test is set of screenshots that show you what your email looks like when it is opened in variety of different email clients (including Windows, web-based, and mobile email clients). You can use design tests to make sure that your email design looks good when it is opened in the email clients that you care about (e.g. popular email clients that your recipients use).
Creating a design test takes only a few seconds and results are available right away.