How to check the mailbox size in Microsoft 365

Do your Exchange Online users keep hitting their Microsoft 365 mailbox size limits? Once a mailbox reaches its quota in Microsoft 365, users cannot send or receive emails until space is cleared. For admins, this quickly turns into repeated administrative work with deleting, archiving, or increasing mailbox sizes after the disruption has already occurred.

By checking mailbox sizes regularly using the following methods, you can choose to increase mailbox size for Microsoft 365 users who need it, apply retention policies where needed, or enforce cleanup well before users hit their mailbox size limits and lose the ability to send mail.

How to check the Microsoft 365 mailbox size in the Microsoft 365 Admin Center

Prerequisites

The Exchange Admin role is applied to the account you use to sign in to the Exchange Admin Center.

1. Log in to the Microsoft 365 Admin Center with an account that has the Exchange Admin role.
2. Navigate to Reports > Usage.
3. Click Exchange and select the Mailbox usage tab.
4. You can view the sizes consumed by your Microsoft 365 mailboxes and other relevant details such as mailbox quotas, archive status, and more.

How to check the Microsoft 365 mailbox size using Exchange Online PowerShell

Prerequisites

Before using Exchange Online PowerShell, please verify that:

• The ExchangeOnlineManagement PowerShell module is installed. If not, install it using this script:
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                                          Install-Module ExchangeOnlineManagement -Scope CurrentUser
                                          Update-Module ExchangeOnlineManagement
                                      

• Connect to the Exchange Online module using this script:
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                                          Connect-ExchangeOnline
                                      

Using Get-MailboxStatistics to export a Microsoft 365 mailbox size report

The Get-MailboxStatistics cmdlet can be used in Exchange Online PowerShell to get detailed statistics of mailboxes, including their sizes, item counts, and last logon times. The syntax for retrieving mailbox sizes is given below:

                                Get-Mailbox -ResultSize Unlimited | ForEach-Object {
                            $s = Get-MailboxStatistics -Identity $_.Identity
                            [pscustomobject]@{
                                DisplayName              = $_.DisplayName
                                TotalItemSize            = $s.TotalItemSize
                                IssueWarningQuota        = $_.IssueWarningQuota
                                ProhibitSendQuota        = $_.ProhibitSendQuota
                                ProhibitSendReceiveQuota = $_.ProhibitSendReceiveQuota
                                ArchiveStatus            = $_.ArchiveStatus
                                StorageLimitStatus       = $s.StorageLimitStatus
                                LastLogonTime            = $s.LastLogonTime
                            }
                            } | Sort-Object @{e={try{$_.TotalItemSize.Value.ToBytes()}catch{0}} -Descending} |
                            Export-Csv -Path "C:\Reports\MailboxSizes-$(Get-Date -Format yyyyMMdd-HHmmss).csv" -NoTypeInformation -Encoding UTF8
                            

Supported parameters

The following table contains some parameters that can be used along with the cmdlet Get-MailboxStatistics to provide statistics on your Microsoft 365 mailboxes.

An example use case for the Get-MailboxStatistics cmdlet

Scenario

An Exchange Online admin notices that some users are complaining about not being able to send or receive emails. The admin suspects this could be due to mailboxes nearing or exceeding their quotas.

This is the cmdlet you will have to run to generate a Microsoft 365 mailbox size report with all Exchange Online mailboxes and their mailbox size statistics.

                                Get-Mailbox -ResultSize Unlimited | ForEach-Object {
                            $s = Get-MailboxStatistics -Identity $_.Identity
                            [pscustomobject]@{
                                DisplayName              = $_.DisplayName
                                TotalItemSize            = $s.TotalItemSize
                                IssueWarningQuota        = $_.IssueWarningQuota
                                ProhibitSendQuota        = $_.ProhibitSendQuota
                                ProhibitSendReceiveQuota = $_.ProhibitSendReceiveQuota
                                ArchiveStatus            = $_.ArchiveStatus
                                StorageLimitStatus       = $s.StorageLimitStatus
                                LastLogonTime            = $s.LastLogonTime
                            }
                            

Microsoft 365 mailbox size reports in M365 Manager Plus

While native admin centers and Graph PowerShell scripting can give you the sizes of your Microsoft 365 mailboxes, filtering through the data in these methods can be a hassle, since you will have to apply these filters every time you run them to get the data that you need. This is where ManageEngine's M365 Manager Plus helps you simplify your Microsoft 365 administration.

M365 Manager Plus is a comprehensive administration and security solution for Microsoft 365. It is used for reporting on, managing, monitoring, auditing, and creating alerts for critical activities in your Microsoft 365 environments.

It simplifies this process by providing a single console where admins can view Microsoft 365 mailbox size reports, quotas, and usage trends at a glance. Instead of juggling scripts or exporting raw data for manual filtering, M365 Manager Plus gives you ready-to-use reports and easy scheduling. You can also drill down into the specific mailbox size details you need with built-in filtering options.

Steps to view Microsoft 365 mailbox size reports in M365 Manager Plus

1. Log in to M365 Manager Plus and navigate to Reports > Mailbox Reports.
2. Go to the Reports tab.
3. Navigate to Exchange Online > Mailbox Reports in the left pane.
4. Click the Mailbox Sizes report. You can view the details you get from the Get-MailboxStatistics cmdlet in this report, without the Graph PowerShell expertise.

Monitor your Exchange Online mailbox sizes and more

M365 Manager Plus’ mailbox size and quota management capabilities help you view, monitor, and control mailbox storage across your Microsoft 365 environment. It simplifies quota tracking, highlights usage trends, and provides proactive alerts; all through an intuitive web console without the need for complex Microsoft Graph PowerShell scripting.

Important tips

Frequently asked questions