If using a full page image (which you should avoid), either use very detailed alt text to convey the meaning of the image or provide a link at the bottom of the email to a website with an alternate version of the email.
Use headings and styles properly.
Don't use tables if possible unless it is for formatting data. If you do use tables, use appropriate HTML markup for the table elements.
Don't use only color to emphasize something or indicate meaning.
Provide sufficient color contrast.
Avoid patterned backgrounds.Make sure any documents you attach to the email are also accessible.