Always make all links fully qualified (e.g. https://www.atomic8ball.com/imagesDesign/logo.png
instead of /imagesDesign/logo.png)
Never use css float, it doesn't work in most versions of Outlook.
If you need to do any sort of positioning or layout, use a <table>
Never use css background images, they don't work in most versions
of Outlook.
Write good alt tags. If a user doesn't select 'Load Images' the
alt tag text is shown.
Along with the prior, specify width and height for images. If a user doesn't select 'Load Images' the space will still be occupied and won't potentially mess up the layout.
Check for non-standard dashes (-) and apostrophes (') when copy-pasting text from
emails or Word docs. These will look different from one that you would type normally
and will show up as a ? in the body of the email.