Well, it's plain text, lacking specific controls for alignment. There is an addon, external editor, that may help, but I have no experience with it. You also face the issue of whatever font the recipient is using. Generally, if you're emailing to private individuals, it's a tossup on whether their email client is set for HTML or not, so messages that are dependent on formatting, may not achieve the objective.

Thanks @david.

It's not about formatting. It's about (as simple as) adding or removing '>' in front of some (block of) text, which is the good-old-indentation marker of email;-) Yes, I can go through the text manually and change the start of a few to 100 lines of quoted email, but there ought to be a simpler way to do this.

I am not worrying on how it is displayed for me (Thunderbird does fine here) nor the receiver (their MUAs also do that right).

NB: right now I copy the text to Emacs, adjust the starts of all lines, then copy back. Not very convenient;-)

PS: I realize how the word "indentation" is unclear, will change the title of the question.

I'm not sure what you're seeking. When I respond to plain text messages with a plain text response, the '>' is inserted at beginning of each line. Please be more specific in what you wish to achieve, as then someone here may have a solution. Thank you.

HI @david, thank you for the feedback. Sorry for not being clear enough.

In "simple terms", I wish to have a command/shortcut to add or remove a '>' from every line in a marked region.

That is, conceptionally, I wish to change the "quote level" of that marked region.

This is relevant for me in various scenarios, e.g.,

• when I realize I want to rewrite text contained in a quote to provide a new version of that text
• when a longer/complicated discussion becomes cumbersome and I want to keep only selected parts of replies from a certain author – and putting these onto one quotation level.
• when the indentation was (partly) wrong in a message that I am now responding to and I want to fix it for clarity of the discussion
• ...

For instance,

> Lorem ipsum dolor sit amet, consectetur adipisici elit, >> sed eiusmod tempor incidunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, >> quis nostrud exercitation ullamco laboris nisi ut aliquid ex ea commodi consequat. > Quis aute iure reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. > Excepteur sint obcaecat cupiditat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum

should be changed to the following – or the other way around

>> Lorem ipsum dolor sit amet, consectetur adipisici elit, >>> sed eiusmod tempor incidunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, >>> quis nostrud exercitation ullamco laboris nisi ut aliquid ex ea commodi consequat. >> Quis aute iure reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. >> Excepteur sint obcaecat cupiditat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum

Hope this is clear:-o) And if not, I hope you keep the passion and let me know where/how I have to clarify more.

PS: It would be the equivalent of macOS Mail.app's CMD-' and CMD-SHIFT-'

Well, you're sorta' playing with fire when you change quoted text or the indicator of quoted text, and there is no easy way to do this while preserving integrity of the overall communication. The simplest way I can advise is the find/replace feature that can go line by line or all to remove the '>' character. Another option is not to select 'reply' but 'edit as new message', which removes '>' characters. Something you might want to experiment with is autohotkey at https://www.autohotkey.com/ . That is a free scripting language that may assist.

Thank you for your feedback @david.

I've been doing this for the last 30+ years for various reasons – to fix up indentation levels. All my previous MUAs could do this, from Gnus (Emacs) to Mail.app ...

Thank you for suggesting external options, but autohotkeys does not seem to be available for macOS. Moreover, it is such an integral part of a MUA that it should be implemented there.

Will put it on _ideas_... :-o (And maybe there are too many details missing in Thunderbird for me to make up for the shortcomings of Mail.app I was trying to resolve by the switch.)