Hello,
I changed the settings in ~/bwdata/env/global.override.env to
globalSettings__mail__smtp__port=25
globalSettings__mail__smtp__ssl=false
globalSettings__mail__smtp__username=
globalSettings__mail__smtp__password=
This works (leaving username/password empty). 
Since Bitwarden is for internal use only (not accessible via internet), that is kind of ok.
Of course it would be better, if port 465/ssl was working.
I hope I get my bitwarden working with my exchange.
Thread closed.