Anyone having success with an online html email builder for notifications?

We are trying to use an online html email builder in our case (unlayer.com) which makes our notification emails look professional, format OK for mobile and desktop easily insert images etc.  However some notifications fail to send.  After opening a case with QB they tell us ...

I can see the outbound email, but there's a rejection from the email server, so it's bouncing back.  The reason given is "550 Maximum line length exceeded" (see RFC 5322 2.1.1).  There's a few Google searches around that. It's a bit technical, but seems to be relates to maximum string lengths not exceeding more than 1000 characters. Essentially like a long string from what I'm gathering (not the content of the entire message). It may be something in that HTML then that is causing this long string in the message formatting, but it only seems as though it may be happening in certain instances since, as noted, the original test record went to the other user.

My question is twofold, First, who is having success using an html email builder that makes it easy to make professional emails that is not having the issues we are having? Maybe we just picked the wrong solution...

Second, does anyone have ninja html skills and know a tool that will identify the line length/string length and is there a way to truncate or get around that issue in the raw html?