I, and others, would like to have the ability to set margins up when composing new emails, instead of having the text sit all the way to the left of the compose email text window. Text margins can be set in most word-processing applications. If this could be done, plenty of people will be happy.
You could use an HTML Template as in this post below via “Menu / Settings / Mail / Templates & Signatures”
Templates & Signatures help documentation
Alternatively, you can submit a Sleekplan suggestion to have margins for others to vote for.