There is no way to indent text (line or paragraph) within a message in the composer. Nothing on the formatting bar, and pressing TAB only takes the cursor to the To line.
Some will probably say this is a feature request, but it shouldn’t be since it’s an important part of rich text formatting of messages.
I don’t think this is a bug - it’s not that something isn’t working as designed, more that you think it should be designed differently. And I do see your point about indentation, although Mailspring’s rich-text editor is not a full-featured text editor in any case (and arguably shouldn’t be). Would you also advocate for spacing between paragraphs, or text alignment, or any of 50 different things?
I hear you. Yeah, I don’t think we need a Word-like full featured experience in the composer, but for emails the basics are necessary, and I would argue that being able to indent a paragraph, to offset a quote or an address or something, is one of them.
Of course, to offset a quote, you use the “Quote” style, which does just that. 
Ah, so it does. Wasn’t aware of that. My muscle memory just has me hitting TAB to indent some text, so the behavior disoriented me.
I don’t think many email clients support tab-to-indent, do they? (FWIW, I was trained out of doing that in documents decades ago, in favour of using proper styles. But that’s a whole 'nother debate! 
)
Certainly most text editors do. But yeah, maybe I just need to get new muscle memory 
I can see how indentation could be replaced with quotes, but being able to insert tabs is a lot more useful than adjusting paragraph spacing - it’s how you can put in a rudimentary table without having to attach a spreadsheet.
When you’re dealing with customers, it looks a lot more professional to put a little effort into having an easily readable, well formatted email - especially if you’re trying to provide some kind of explanation for something technical, in a non-technical way.