Ranked in these QuestionsQuestion Ranking
Pro Great documentation
The documentation is interactive and very thorough for such a new project.
Pro Free and open source
That's the best you can have.
Pro Online editor
You can work in online editor and export result from code to html to send email.
Pro Modular approach
You can build new modules or choose existing. All can be mixed to create responsive emails.
Pro There is integration with PHP using online editor
You can connect existing app to use this service and have up to date converter.
Con Some effort need to be added to adjust email
For example landscape mode in iOS Email on iPhone.