So I am quite confident now, that there is something wrong with the line wrapping.
I grep-ed through all files (grep -r wrap *), and found the two functions doing the wrapping:program/include/rcube_shared.inc:
function rc_wordwrap($string, $width=75, $break="\n", $cut=false)
program/steps/mail/func.inc:
function rcmail_wrap_quoted($text, $max = 76)
Once again, the problem is, that at first in the editor when replying everything looks OK, but once sent, quoted text gets line wrapped a second time, and some lines always get messed up.
If normal wrapping is done at a line length of 75 characters, shouldn't it be 77 for quoted text, since the quotation inserts two additional characters (">" + space)?
I would so much appreciate, if someone could post a fix for that problem.