Difference between application impersonation and Mailbox delegation:
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Learn How To Assign Mailbox Delegation Permissions in Office 365.
Watch the complete video and easily learn how to grant mailbox delegation permissions to Office 365 Mailbox.
Watch video and practically learn how to assign Send as, Send on Behalf and Full access permissions in Office 365.
Send As permissions: Send email as some other users.The recipient will never know if you have sent this mail or the user has sent.
Send on Behalf: With this, you can send email to someone on behalf on some other mailbox.
Full Access Permission: With this permission you can even open someones mailbox if you have permissions.
