Chose „users and groups“ in the left panel view and setup a new user by clicking the + symbol.
Provide First Name, Last Name and User Name for the new mailbox user.
Our recommendation: RdxMaildepotImporter.
Confirm the settings with “next” and DO NOT assign a adminstrator role in the next step.
Confirm with “next” and assign the Exchange Online license.
Assign the role ApplicationImpersonation to the created user (RdxMaildepotImporter)
To do so, select „permissions“ → admin roles in the Exchange admin Center
Double click the role „ApplicationImpersonation“
Here you add the created user (example „RdxMaildepotImporter“) in the lowest Field „Members“ as a member of the ApplicationImpersonation role.