PutsMail VS Mosaico.io

You can test out the html using something like PutsMail or Testi@. If you like the result, rename it the file to IntegrationTests/Snapshots/Welcome.expected.html. - Source: dev.to / over 2 years ago

I dont know if this helps, its still like how people used to code layouts before flexbox/grid, so you have to have some knowledge of table layouts, and you have to check support in all email platforms just like we check for browser support.. I used to use putsmail.com. Source: over 2 years ago

Then use something like this to test them Https://putsmail.com/. Source: over 2 years ago

You can test your markup using https://putsmail.com/. Source: about 3 years ago

In summary, emails present unique challenges, yet they can still exude aesthetic appeal. Now, while I made some jokes about irritating your UX team, it is imperative that you as a developer can effectively communicate these restrictions to them so that they can best create a design that is beautiful and possible to make. This collaborative approach ensures a balance between aesthetics and functionality. Explore... - Source: dev.to / 5 months ago

Maybe you could integrate something like https://mosaico.io/ or https://grapesjs.com/demo-newsletter-editor.html. Source: over 2 years ago

Never tried but have a look at Mosaico and GrapesJS. Both are open-sourced and used by Mailtrain. Source: about 3 years ago