A. Use short sentences and paragraphs.
B. Use design elements like spacing and dividing lines to distinguish the content sections from one another.
C. Use bold typeface and sub-headers to make certain words stand out.
D. Use bullet points to showcase benefits.
E. Use web-safe standard fonts. For Eg. Arial, Arial Black, Arial Narrow, Comic Sans, Courier New, Georgia, Impact, Tahoma, Times New Roman, and Verdana. However, there are ways to setup non-standard fonts in an email by providing one of the web-safe fonts as fallback font.
F. Ideal font size for body copy is 14 pixels and title is minimum 22 pixels which provides a decent readability on mobile phones.
G. Double check your copy for spellings and grammar.