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