After providing the service accounts access to mailboxes, you must verify the permissions by using Application Check Readiness.
You must verify permissions only for Exchange Online service accounts.
Exchange Online service account must have Exchange administrator permissions.
Application Check Readiness validates the following information:
Service account email address and password
Azure application permissions
Service account impersonation permissions
In the navigation pane, go to Protect > Applications > Exchange.
The Exchange page appears.
Right-click the Exchange client, and then on the upper-right of the page, click Settings.
The Exchange settings page appears.
In the Infrastructure settings section, under Client readiness, click show details.
The Check Readiness page appears.
From the Check readiness on list, click the Application check check box.
In the upper-right of the page, click Check readiness.