Litmus is one of the most widely-accepted design and spam testing services for web and email. Litmus is integrated with Campaign Monitor out-of-the-box to provide spam testing and inbox previews.
Litmus testing is built right into Campaign Monitor so you can design your email and test it for spam and formatting before you send or schedule your campaign.
Litmus spam tests and inbox previews are a paid feature for Campaign Monitor customers on the Basic and Pay-per-campaign plans. They’re included for free as part of the Unlimited and Premier plans.
Litmus testing is automatically included in the Campaign Monitor app. To start using it, log in and start testing your emails from the Campaigns tab.