In my experience, DTRegister is particular about what you can and cannot include in the emails.
I have found the following to work:
Styled elements (example):
<p><span style="font-size:20px;font-weight:bold;color:#DF9521;font-family:arial;line-height:110%;">
Tables
Paragraph and Span tags
Images and URLS[/ul]
I have found that these elements will not work
HTML Header and Body Declarations
Stylesheets
Inline styles in the head area
Generally, the basics work.
I have trouble getting tables to display with a vertical align top in Entourage. My tables have white space above them. This issue basically renders tables useless.
You really have to do a lot of trial and error but you can get a decent looking email; just likely not what you want. The email will be very basic.
____________________________________________
******** Be sure to turn off any text editors such as JCE, RoxPad, etc when you are pasting Code into the email windows. You can turn these editors off in Joomla from Site > User Manager > (select user) > User Editor > (select: Editor - No Editor)
Don't get me wrong, you will still encounter the limitations I mentioned above, but setting the User Editor to "No Editor" will reduce your chances of problems.