I'm replying on this with a very long line which is then wrapped and space-stuffed because the draft is saved as format=flowed.
From what's specified in RFC 2646 some lines need to be space-stuffed to avoid munging during transport.

X

On XX.YY.YYYY Y:YY, Somebody wrote:
> This part is a reply without any flowing lines. rcube_mime::unfold_flowed()
>> has to be careful with empty quoted lines because they might end with a
> space but still shouldn't be considered as flowed!
> 
> The above empty line should persist after unfolding.
> xxxxxxxxxx. xxxx xxxxx xxxxx xxxx xx xx.xx. xxxxxx xxxxxxxxxxxx, xxxx xx
> 
> ... and this one as well.

> > text

-- 
Sig
