Hello @wALF ,
My suggestion would be work with adobe deliver-ability team to get an idea of how the emails are performing with Microsoft in terms of deliver-ability and where your IP/Domain reputation stands now.
On the basis of all this, Adobe team recommend you to increase/decrease the number of emails you are sending out to Microsoft Email addresses.
Also, you will have to consider that most of Microsoft users will be using outlook client and if you emails are not rendering properly there, then they are landing in the Junk/spam folder hence the open rate is very low.
Thanks