because people in my business are very active in sending the whole history in EACH mail,

Unfortunately that quoted history is not easily navigable and doesn't include attachments.

there is a setting with the server-confg, which defines what is to be done with send mail

I looked into it, but it unfortunately only applies to future messages sent. Typing this I just realized that I could maybe move my existing sent messages into the uber-folder, I'll give that a try.