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Typically, the amount of mailbox storage available is determined by the mailbox type and the user's subscription license. For instance, if a user has subscribed to Office 365 Enterprise E1, then the maximum storage available to the user mailbox is 50 GB and archive mailboxes is 50 GB. On the other hand, if a user has subscribed to Office 365 Enterprise E3, then the maximum storage available to the user mailbox is 100 GB and archive mailboxes is Unlimited.

Depending upon how every type of mailbox is used, administrators can reduce the size of the mailbox either per user or globally.

If for instance, a user mailbox runs out of storage space, then that mailbox will not be able to accept any more emails. To avoid this, administrators can configure Exchange Online to send out different types of notifications, namely - Warning, Prohibit Send, and Prohibit Send/Receive - to users, depending upon the usage of their mailbox and how soon its storage space will be exhausted. Such notifications prompt users to clear up storage space, so that they can continue using their mailboxes without any interruption.

Besides the individual users, administrators also need to keep tabs on how the users use the different mailboxes - eg., user mailboxes, archive mailboxes, clutter enabled mailboxes, etc. This will help administrators capture a potential storage space crunch much before it actually occurs and affects user productivity. Also, this may enable administrators rapidly identify users who are over-utilizing the storage space available to them. Administrators can in fact alert such users, even before Exchange Online sends out notifications! This is exactly what the EXOMailBoxStatsTest test helps administrators perform!

This test monitors the usage of the user mailboxes, and proactively alerts administrators if these mailboxes exhibit abnormal growth trends. Detailed diagnostics of this test lead administrators to those users with large-sized mailboxes, thus enabling them to accurately identify which user is over-utilizing the storage space available to them, so that they can warn such users of the impending space contention and urge them to take appropriate action. Additionally, the test also alerts administrators whenever Warning, Prohibit Send, or Prohibit Send/Receive notifications are sent out to any user. Detailed diagnostics provided by the test pinpoint the users who have received such notifications, so that administrators can intervene and ensure that such users quickly clear up storage space and do not cause mail traffic to be blocked. The usage of archive, inactive, and clutter enabled mailboxes are also monitored and administrators alerted if such mailboxes show signs of over-utilization.

Outputs of the test : One set of results for the monitored Offce 365 tenant.

Measurement	Description	Measurement Unit	Interpretation
Total_Mailbox_Size	Indicates the total mailbox size across all user mailboxes.	Number	Use the detailed diagnosis of this measure to view the top-10 users, in terms of their mailbox size.
Mailboxes_Growth_Rate	Indicates the percentage growth in mailbox size.	Number	This measure is computed using the following formula: [{Total mailbox size in the current measurement period (-) Total mailbox size in the last measurement period} / Total mailbox size in the last measurement period]100* If the value of this measure is increasing consistently and is rapidly approaching 100%, it is a clear indication that one/more users are over-utilizing their mailbox storage. To know who those users are, use the detailed diagnosis of the Total mailbox size measure. This will point you to the top-10 users with large-sized mailboxes.
Mailboxes_Over_Quota	Indicates the number of user mailboxes, the storage space of which has been consumed beyond the Warning limit configured.	Number	A user receives a Warning notification by email if his/her mailbox is approaching the maximum size limit. This warning is intended to encourage users to delete unwanted mail. To know which users' mailboxes are about to run out of storage space soon, use the detailed diagnosis of this measure. The current size of each mailbox and the Warning quota set for that mailbox is also displayed, so that administrators can understand how soon each mailbox will be running out of space.
Inactive_Mailboxes	Indicates the number of inactive mailboxes.	Number	When an employee leaves your organization (or goes on an extended leave of absence), you can remove their Office 365 account. The employee's mailbox data is retained for 30 days after the account is removed. During this period, you can still recover the mailbox data by undeleting the account. After 30 days, the data is permanently removed. But if your organization needs to retain mailbox content for former employees, you can turn the mailbox into an inactive mailbox by placing the mailbox on Litigation Hold or applying an Office 365 retention policy to the mailbox in the Office 365 Security & Compliance Center and then removing the corresponding Office 365 account. The contents of an inactive mailbox are retained for the duration of the Litigation Hold placed on the mailbox or the retention period of the Office 365 retention policy applied to it before the mailbox was deleted. You can still recover the corresponding user account for a 30-day period. However, after 30 days, the inactive mailbox is retained in Office 365 until the hold or retention policy is removed. By observing the variations to the Inactive_Mailboxes_size measure over time, you can figure out if your inactive mailboxes are growing in size abnormally and draining valuable storage space. If this is the case, then you may want to change the hold duration of the Litigation Hold or Office 365 retention policy applied to the inactive mailbox, or completely remove the Litigation Hold or retention policy applied to that mailbox. This will ensure that the email contents are not retained for much longer, and are instead, deleted permanently. This will release storage space.
Inactive_Mailboxes_size	Indicates the total size of inactive mailboxes.	GB
Archive_Mailboxes	Indicates the number of archive mailboxes.	Number	An archive mailbox is a specialized mailbox that appears alongside the users' primary mailbox folders in Outlook or Outlook Web App. Users can drag and drop messages from .pst files into the archive, for easy online access. Users can also move email items from the primary mailbox to the archive mailbox automatically, using Archive Polices, to reduce the size and improve the performance of the primary mailbox. Users can restore items they have deleted from any email folder in their archive. When an item is deleted, it is kept in the archive's Deleted Items folder. It remains there until it is manually removed by the user, or automatically removed by retention policies. If the Archive mailboxes size measure reveals that the archive mailboxes are growing in abnormally in size, it could mean one or both of the following: User mailboxes are rapidly running out of space, owing to which many emails have been moved to the archive mailboxes, thereby eroding the archive space; The Deleted Items folder of the archive mailboxes is rapidly filling up, but is not being emptied at the same pace either manually or by the retention policy To make more space in the archive mailboxes, you may want to consider manually removing emails from the mailboxes or their Deleted Items folder, or change the retention policy of the Deleted Items folder.
Archive_Mailboxes_Size	Indicates the total size of archive mailboxes.	GB
Clutter_Enabled_Mailbox	Indicates the number of clutter-enabled mailboxes.	Number	The idea of the Clutter feature is to take “low-priority” emails and automatically move them out of your inbox into another folder. The thought is if there is a particular type of email you rarely read, but isn't junk/spam, the message will be filed away into a folder where you can review it later. In order to achieve this goal, Exchange Online needs to be able to watch your behavior for a period of time before it can be “trained” on what to identify as clutter. Clutter is now enabled by default for all mailboxes. Users can disable Clutter themselves via “Options” within OWA. Otherwise, administrators can disable Clutter via PowerShell with the “Set-Clutter” cmdlet.
Clutter_Disabled_Mailbox	Indicates the number of clutter-disabled mailboxes	Number
DbSndQte	Indicates the number of user mailboxes, the storage space of which has been consumed beyond the Prohibit send limit configured.	Number	A user receives a prohibit-send notification email when the mailbox size limit is reached. The user cannot send new messages until enough email is deleted to bring the mailbox below the size limit. To know which users' mailboxes have reached their prohibit-send limit, use the detailed diagnosis of this measure. The current size of each mailbox and the Prohibit Send quota set for that mailbox is also displayed, so that administrators can understand why the Prohibit-send notification was sent. Administrators can then urge the concerned users to quickly clear up space in their mailboxes by either manually deleting mails or moving mails to archive mailboxes.
DbSndRecQte	Indicates the number of user mailboxes, the storage space of which has been consumed beyond the Prohibit send/receive limit configured.	Number	A user receives a prohibit send/receive notification email when the mailbox size limit is reached. Subsequently, Exchange Online rejects any incoming mail when the mailbox size limit is reached, and sends a non-delivery report (NDR) to the sender. The sender has the option to try resending the mail later. To receive messages again, the user must delete email until the mailbox is below the size limit. To know which users' mailboxes have reached their prohibit send/receive limit, use the detailed diagnosis of this measure. The current size of each mailbox and the Prohibit Send/Receive quota set for that mailbox is also displayed, so that administrators can understand why the Prohibit Send/Receive notification was sent.
ArrMailsExWnQta	Indicates the number of archive mailboxes, the storage space of which has been consumed beyond the Warning limit configured.	Number	To know which archive mailboxes are about to run out of storage space soon, use the detailed diagnosis of this measure. The current archive size of each mailbox and the Archive Warning quota set for that mailbox is also displayed, so that administrators can understand how soon each archive mailbox will be running out of space.
MbNearRecvrQuota	Indicates the number of user mailboxes, the storage space of which has been consumed beyond the recoverable items limit configured.	Number	The Recoverable Items folder contains items deleted by Microsoft Outlook and Microsoft Office Outlook Web App users or by the Mailbox Assistant. The duration that deleted items remain in this folder is based on the deleted item retention settings configured for the mailbox database or the mailbox. By default, a mailbox database is configured to retain deleted items for 14 days, and the Recoverable Items warning quota and Recoverable Items quota are set to 20 gigabytes (GB) and 30 GB respectively. However, if In-Place Hold or Litigation Hold is enabled for the mailbox, the Recoverable Items folder can accumulate deleted items beyond the specified retention period and can also maintain different versions of modified mailbox items. To know which users' mailboxes have reached their recoverable items limit, use the detailed diagnosis of this measure. The current size of the deleted items, the count of deleted items and the recoverabe items quota set for that mailbox is also displayed, so that administrators can understand which mailboxes are nearing recoverable items quota.