The default size of Office 365 mailboxes varies based on the mailbox type and user subscription. The default size of a user mailbox is either 50 GB or 100 GB, and when the space is exhausted, users won't be able to send or receive emails. To avoid this, admins need to keep a check on the mailbox size.

For this purpose, Microsoft 365 Admin Center provides Mailbox Usage report. (Microsoft 365 Admin Center -> Reports -> Usage -> Select a report -> Exchange -> Mailbox Usage)

The Mailbox Usage report provided by the Admin Center shows only the storage space consumed by user mailboxes. To view the actual user mailbox size, shared mailbox size, resource mailbox usage, and other mailbox related details you have to navigate to the Exchange Admin Center (EAC).

O365 Manager Plus' Mailbox Size reports

O365 Manager Plus has a dedicated set of reports to gain insights on mailbox usage. Some of the reports include,

• Mailbox size
  

• Mailbox size restriction
  

• Current mailbox size vs quota
  

• Archive mailbox size
  

• Mailbox storage information
  

• Shared mailbox size 
  

• Room mailbox usage
  

Apart from the aforementioned reports, the tool also provides reports that provide every day mailbox usage statistics. Unlike Microsoft 365 Admin Center, O365 Manager Plus reports on all types of mailboxes from a single window. 

Steps to view the Mailbox Size report

1. Login to O365 Manager Plus.


2. Go to the Reports tab.


3. Navigate to Exchange Online -> Mailbox Reports in the left pane.


4. Under the Size Reports category click on Mailbox Size report.

 

Note: To view Shared Mailbox Size and Room Mailbox Usage reports navigate to Exchange Online -> Other Exchange Reports in the left pane.

O365 Manager Plus vs Microsoft 365 Admin Center 

• Pre-defined reports: O365 Manager Plus provides more than 100 pre-defined reports on Exchange Online mailboxes while the Admin Center provides relatively few reports.
  

• Data older than 180 days: Reports provided by Microsoft 365 Admin Center does not show data older than 180 days. In O365 Manager Plus all the data right from the day of installation can be viewed without any restriction.
  

• Customize reports: O365 Manager Plus allows you to create your own report based on your organizational requirements. However, the Admin Center lacks such flexibility.
  

• Schedule reports: All the reports of O365 Manager Plus can be scheduled to be generated at specified intervals and have them delivered to your inbox via email.
  

• Export reports: O365 Manager Plus exports reports to CSV, XLS, PDF or HTML formats, while the native portal exports only to CSV format.