The Premailer tool verifies CSS in your emails and places it inline so it’s more HTML-email friendly. You can also test your CSS in standard email applications and receive recommendations about how to improve before you send your campaign.
Testing with Premailer is available from within the Email Campaign Builder. Simply click on Test your campaign on the left hand side of the campaign builder and select Premailer
Click on Run the test to begin.
You will immediately see a summary of your code and how it will show depending on the different email clients, and whether it is safe, risky or poor to use.