Does mailwizz offer any type of design validation making sure that you're design will work on all email clients. Mailchimp and Constant Contact offer this but you need a subscription.
There are businesses making a living out of this, such as litmus, that's where you should test your templates before uploading them in mailwizz.
Just a heads up, the free email templates we offer at https://www.mailwizz.com/free-email-templates/ are all tested with litmus.