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No need to register an account. You can use your existing Microsoft Login.
The App Registration will need the following Microsoft Graph Application Permission:
User.Read User.Read.All Mail.Send Mail.ReadWrite ServiceHealth.Read.All
<aside> ℹ️ Administrators who want to limit the app's access to specific mailboxes can create an application access policy by using the New-ApplicationAccessPolicy PowerShell cmdlet. https://docs.microsoft.com/en-us/graph/auth-limit-mailbox-access
</aside>
This mailbox will be user for monitoring. It should not be a mailbox that is active used by a user, as it will receive email regularly.
We send the monitoring data to your Log Analytics Workspace. So the data stays in your tenant and you can keep the monitoring data as long as you want.
Based on your needs you can setup alert when emails are not delivered or delayed.