MaxBulk Mailer has absolutely nothing to do with the kind of contents you use, it is just a bulk emailer. What you write/paste in her message panel is sent as is. The very only compatibility problem you will have is with a browser or a mail reader software. The MaxBulk Mailer preview is made with WebKit, the Safari HTML rendering engine.
Hi,
I see this post is quiet old but I have the same problem now in 2023 : a page made in html5, visible with safari, is not previewed in Maxbulk or in my mail software after I send it. I can't understand what's wrong.
Auto-generated code uses to be created for websites and will be rendered by web browsers. Emails will be rendered by email clients as well, so it is better to use clean code. HTML5 is the best HTML norm either along with CSS3. Previous norms are terrible. MaxBulk Mailer doesn't care since it sends the code as is. The preview is done with an embedded browser renderer.
Out of curiosity, my site, www.maxprog.com, is coded by hand with a text editor (BBEdit). It is HTML5+CSS3 with lots of PHP.