Some Emails Design Guidelines are Discussed here
The first rule of email designing is be transparent while writing an email. The content Should be simple and transparent so that receiver get your point What you exactly want to say.
Avoid such sentences that doesn't make any sense.
Do not write too much go for simple email.
Always add a subject in the email.

Avoid Adding Video and GIF in the email.
Add headings if you are writing a long content.
Put your logo in email so that the Receiver knows where it come from.