Hi,
After contacting the help desk, there was currently no option to configure a shared Office 365 mailbox for the bounce configuration.
If you have the same problem, this is the solution;
Edit this file: back/helpers/bounce.php
Add this code after rule: 212
$ email = trim ($ this-> username);
// test if string contains '\'
if (strpos ($ email, '\')! == false) {
list ($ user, $ authuser) = explode ("\", $ email);
list ($ x, $ domain) = explode ("@", $ user);
$serverName.='/authuser='.$user.'/user='.$authuser.'@'.$domain;
}