Alert: Mailbox Backups Failure "M365 Mailbox has exceeded storage limit"

Symptoms

M365 Mailbox backup tasks fail and the following alert is generated, 

"M365 Mailbox has exceeded storage limit"

Root Cause

Below are some of the common causes for this failure.

• The storage quota assigned to a mailbox as per the license is exhausted
• The storage quota assigned to the mailbox by the M365 Admin is exhausted

Refer to the following article from Microsoft to understand quota assignments. 

https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#mailbox-storage-limits 

Resolution

• Increase the storage quota of the mailbox
• Reduce the mailbox size by clearing out/archiving older mails.

Note: Mailbox backup tasks will continue to fail until these steps are taken

 

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