We try to talk with our customers via email only from odoo tasks, customers dont have odoo account. 

In normal mail conversations when you reply to something original message is attached in text form and client knows what he have written and basically whole conversation history is in 1 message.

Now with current odoo behavior our customers keeps get confused to what we replied because subject with Re: is not enough. With some clients this would be fine for example gmail, with others no thunderbird. I havent tested with others.

I was very surprised that noone complains about this. I tried to google this in few different ways, but couldnt find anything maybe I m doing something wrong any ideas?

PS I tried "keep original" setting in "incoming mail servers". But it attaches it as .eml and in thunderbird is shows up as an attachment.