Do you really think those reading your mail on 4" phone screen will appreciate the extra scrolling they need to do because you message does not start on the left of the screen.

I have a desk computer with a 22" HD LCD monitor, plenty of space for an indentation in a Email body. Most of the people I know don't read their Email on a little phone screen & I don't even have a cell phone. If I knew of anyone reading their Email on a cell phone, then an indentation could be eliminated for easier reading.

Intents are a hangover from the printed word. They have no place in email really. That is my opinion, but I had no idea why so I spent a little time googling and found this article. Which explains things beautifully.

http://www.prdaily.com/Main/Articles/Why_dont_writers_indent_paragraphs_anymore_18235.aspx

Thunderbird prepares mail in paragraph mode by default and places the space between paragraphs, the there is no lexicographical reason to have indentation of the first line.

As for setting a margin, which does appear as a request from time to time, There is none in the digital world of HTML. Basically without a sheet of paper there is no reason not to put text in all of the relevant space. It is not as if it makes reading it harder.

But as for saving the settings for indent level or a left margin. I am not aware of anything of that type. All I can suggest is you create a template that have the document formatting you like and use that when composing new messages rather than clicking write.

This add-on may help in the pursuit of building a collection of custom templates. https://addons.mozilla.org/en-US/thunderbird/addon/smarttemplate4/

Especially when used in conjunction with this add-on https://addons.mozilla.org/en-US/thunderbird/addon/stationery/?src=ss