When replying by mail, Discourse set a line break every 70 characters or so.
Is it made on purpose ? It’s quite unsightly.
AI-generated summary
Users are discussing an issue with line breaks when replying to Discourse via email. thomasschmit initially thought it was a Discourse issue, but zogstrip pointed out that it’s likely due to the email client. thomasschmit checked the sent messages and didn’t see line breaks, but Mittineague explained that line breaks can be hidden and may be caused by different operating systems using different line breaks. thomasschmit discovered that Discourse uses the plain text version of the email, which includes line breaks, rather than the HTML version. zogstrip confirmed this. However, codinghorror mentioned that this behavior will change, and Discourse will begin to prefer the HTML version to avoid the line break issue. sam shared a similar issue on another forum and suggested that the CommonMark project could enable traditional Markdown line breaks to resolve the issue.
It’s not Discourse that is doing it but your mail client 
Really? I use Gmail and when I look at the sent messages, there is no line breaks.
They aren’t so easily seen.
Some OS use “\r” some “\n” some “\r\n”
This can be fixed with “replace” code. But I don’t know if that’s what’s happening here.
Hum… When I look at the source of my mails, there is indeed line breaks in the plain text version.
Is Discourse using this one rather than the HTML one, which may then be parsed in Markdown?
That’s exactly what’s happening (we always prefer the text version to the HTML version).
This is changing, we will begin to prefer HTML version as the line break problem is impossible otherwise.
I still see a similar issue regularly here:
I wonder if the folks at commonmark should just go ahead and just enable traditional markdown linebreaks though.