Email template is not showing up in Outlook.com. This was an annoying problem to try to diagnose. Finally found the
answer here. Remove any <HTML> commentsor tags from <style>.
The standard default Marketo template come with:
/* This most probably won't work in all email clients. Don't include
<code _tmplitem="35" > blocks in email. */
You will need to remove this line or replace
<code _tmplitem="35" > with CODE.