HotDocs might be a bit more popular than Unlayer. We know about 1 link to it since March 2021 and only 1 link to Unlayer. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
Before that we had some luck using RazorLight, but we did all email templates in an editor Unlayer, exported the html and made a layout and view files. Source: about 1 year ago
There are others like hotdocs.com or xpertdoc.com but I've never checked them out. Source: about 3 years ago
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