When sending email from i.e. Visma or other 3. party applications you need a SMTP server. Some times you can use the local ISP SMTP server, but then you need to add that SMTP server to yours domain SPF record. You might don`t wnat to do this.
Here is the solution:
Log in to the tennant, and start Exchange management console.
Create a connector filtering on the public IP address beeing used for the server that hosts Visma.
In Visma add a SMTP server using your MX record for that tennant (i.e. itstyring-no.mail.protection.outlook.com ) and port 25.
This connector will relay email with from addresses containing a valid domain for that tennant.