Litmus
As one of the most widely-accepted design and spam testing services for web and email around, Litmus was an obvious choice when it came to selecting a provider for our in-app testing tools.
The great thing is, Litmus just doesn't sit still. So when they add a new email client to their testing suite, we make it available to you, meaning that you can access the full Litmus experience without needing to sign up for a new account, or leaving Campaign Monitor. In collaboration with Return Path, we use Litmus to provide you with the most comprehensive email test results, in the shortest time possible.
