When writing a new message, every line that begins with a space character, will get an extra space in front of it. So X is X but X becomes X and X becomes X This way it is impossible to write only one leading space in a line. Also you always need to remove one space from your ASCII art signature or whatever in order to get it displayed correctly on the other end. The problem only seems to reside in the message composition. A draft is already stored with 2 spaces as is the message put in the Sent folder after sending it. The source code is consistent with what is displayed then. So when composing the message, the extra space is added in the first instance. This problem already exists for a very long time, at least since 0.8.
Yep, it does that; it's part of how format=flowed works. See bug 168420 for information about f=f, including the informative FAQ attached to that bug. *** This bug has been marked as a duplicate of 141983 ***
Oh, thanks for the link. I didn't expect such a specification behind the issue. Deactivated that flowed stuff entirely now, it seemed to be deactivated only half (the display part) which caused the inconsistent behaviour withing the application itself. But IMO the flowed stuff is more a pain than it was supposed to help anything. (What should it actually help?) It requires the user to expect this behaviour, has some interesting but broken features and might cause a wrong/unintended display on the other side. (What clients do actually support f=f?) I'd recommend anyone writing plaintext mails to disable this 'feature' by default.