Just in the last year, I have sent over 30 million emails to our users. Mostly plain text. The last two emails, to the list with over 400k subs, were sent using MJML. Anecdotally OR dropped ~3-5 points. OR avg on the above numbers was 33%, rarely dropping below 30%. Now was 30% in both cases, but it took longer to get to that (I suppose the cause of email filters introducing some delay). Also, our links in the email received spikes of traffic (email spam software running crazy over them). Also, I've got some replies from happy users on how nice the new emails are. I yet have to see what to conclude. TLDR just use MJML.
