Hello Björn and thank you for reaching out to us with your enquiry.

Please check these settings are as below:

in Module: Mail and Conferences > Settings > Email SMTP server

SMTP adress: smtp.office365.com
Port: 587
Encryption method: STARTTLS

user address: postmaster@mydomain.com
password: your Office365 admin password

best regards,

Giuseppe
HansaWorld Support