When replying to a plain text mail from the sogo web mail with html compositing enabled, quotation in plain text is broken, whereas the html looks fine:
| this is a test mail
answering to this plain text mail with html compositing leads to:
| > this is a test mail
| answer
in the html part and:
| this is a testmailanswer
in the plain text part.

Answering this mail again with web mail, leads to wrong quotation in the html part too:
| >this is a test mail
| answer
| again answering
when this should actually be:
| >> this is a test mail
| > answer
| again answering

In case you need more information, I could provide either screenshots or mails, but the issue should easily be reproducible with the sogo demo.