You are repeating the very same history and excuses (from the IT Dpt.) I lived and heard. Davmail works for me on Linux and MacOS even from outside the intranet. Give it a try and I would be happy to help, I have a gmail account with my nick.
Edit: night time here now, sorry if I cannot help right away.
Thanks! Then I’ll give that a shot and email you if I get stuck :)