Use In-line CSS.
Keep the layout simple.
Make sure your email can tell your
story WITHOUT graphics, as all modern email clients suppress graphic
images. Have your graphics add value, when the reader hits their
“Show Images” button.
Provide a subject line that will
grab the readers attention.
Some modern email clients have
“dumbed down” their ability to read and interpret HTML
(I am looking at you, Microsoft Outlook), so the simpler you keep
your email, the more likely it is to reach your audience intact.