Identify Exchange Online quota exceeded mailboxes

Regularly monitoring mailbox sizes and their assigned quotas is a critical administrative task for managing mailbox storage and ensuring service continuity. When Microsoft 365 mailbox storage limits (previously Office 365 mailbox storage limits) are unmonitored, users eventually hit their Outlook email size limit. At that point, they can’t send or receive emails.

Regularly reviewing Microsoft 365 mailbox size limits lets you identify users who need more space, enforce retention policies where necessary, and prompt cleanup actions before anyone hits storage limits and gets blocked from sending emails.

This guide explains how you can review whether your mailbox quotas have been exceeded and their current status with the Microsoft 365 admin center, PowerShell, and most efficiently, with ManageEngine M365 Manager Plus, a dedicated Microsoft 365 administration tool.

What are Microsoft 365 mailbox size quotas?

Mailbox quotas define how much storage each user can use before email delivery stops. Increasing the quota within the Exchange Online plan allows users to continue sending and receiving mail without an add-on or additional licenses.

An admin will apply mailbox quotas to prevent storage misuse and maintain service performance. These limits vary based on license type but generally include:

1. Issue Warning Quota: The point at which the user receives a warning that their mailbox is nearing capacity.
2. Prohibit Send Quota: The user can no longer send new emails but can still receive them.
3. Prohibit Send/Receive Quota: The mailbox can no longer send or receive any emails.

These quotas are the technical upper limits set on an individual mailbox.

Method 1: How to get a list of mailbox quotas using the Microsoft 365 admin center

Prerequisites

You need the Exchange Administrator role for the account you use to sign in to the Microsoft 365 admin center.

Steps

1. Log in to the Microsoft 365 Admin Center.
2. Navigate to Reports > Usage.
3. Click Exchange and select the Mailbox usage tab.
4. Click Choose Columns, select the mailbox quota columns you want to view, and click Save.

   

5. Click Export to get a report of your Microsoft 365 mailbox quotas in a CSV file.

Limitation

You can view all mailbox size quotas in this report, but you cannot schedule it as a recurring report. This forces you to check it manually each time, which is inefficient and cumbersome.

You can get around this limitation by using the Schedule Reports feature in M365 Manager Plus to notify you of mailboxes that have exceeded their quotas and generate your reports every week automatically, without you having to manually sift through them to separate the mailbox quota exceeded entries.

Method 2: How to get a list of exceeded mailbox quotas using Exchange Online PowerShell

PowerShell offers the most comprehensive way to report on mailbox size and detailed mailbox quota settings, which is essential for proactive management when users face an Outlook mailbox size that's exceeded its limit.

Prerequisites

Before using Exchange Online PowerShell, verify that:

1. The Exchange Administrator role is applied to the account you use to sign in to Exchange Online PowerShell.
2. You are connected to the Exchange Online PowerShell module.
   1. To check if the Exchange Online PowerShell module is installed, use this script:
      Copy

                                              Get-Module -ListAvailable ExchangeOnlineManagement
                                          

   2. If it does not return a value, you will have to install the module. To install the Exchange Online PowerShell module, execute this script:
      Copy

                                              Install-Module ExchangeOnlineManagement -Scope CurrentUser
                                          

   3. To connect to Exchange Online PowerShell, run this script:
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                                              Connect-ExchangeOnline
                                          

Using Get-Mailbox to export mailbox quotas in Exchange Online

The Get-Mailbox cmdlet can be used to retrieve a list of all mailbox size quota properties.

Use this syntax to retrieve the quota setting for all mailboxes:

Get-Mailbox -ResultSize Unlimited | 
Select-Object DisplayName, PrimarySmtpAddress, IssueWarningQuota, ProhibitSendQuota | 
Export-Csv -Path "C:\Reports\MailboxQuotas-$(Get-Date -Format yyyyMMdd-HHmmss).csv" -NoTypeInformation -Encoding UTF8
                            

Using the new Get-EXOMailbox cmdlet per Microsoft's recommendation

Microsoft recommends using Get-EXOMailbox instead of Get-Mailbox in Exchange Online. The Get-EXO cmdlets are optimized for the cloud service, offer better throttling management, and will continue to be updated, whereas the older cmdlets are maintained mainly for backward compatibility.

Use this syntax to retrieve the mailbox quotas in Exchange Online with the updated Get-EXOMailbox cmdlet:

Get-EXOMailbox -ResultSize Unlimited -Properties IssueWarningQuota, ProhibitSendQuota | 
Select-Object DisplayName, PrimarySmtpAddress, IssueWarningQuota, ProhibitSendQuota | 
Export-Csv -Path "C:\Reports\MailboxQuotas-$(Get-Date -Format yyyyMMdd-HHmmss).csv" -NoTypeInformation -Encoding UTF8
                            

Supported parameters

The following table contains some parameters that can be used with the Get-Mailbox and Get-EXOMailbox cmdlets to provide details on your mailbox quotas.

An example use case for the Get-EXOMailbox cmdlet: Find mailbox quotas of all mailboxes in your organization

Scenario: An IT administrator needs to find all users who have already exceeded their warning quota to manage storage proactively and prevent mailbox quota exceeded errors. This requires comparing the current size (from Get-EXOMailboxStatistics) with the quota limit (from Get-EXOMailbox).

This is the cmdlet you will have to run:

$report = @()
$mailboxes = Get-EXOMailbox -ResultSize Unlimited -Properties IssueWarningQuota

foreach ($mbx in $mailboxes) {
    # Get current size
    $stats = Get-EXOMailboxStatistics -Identity $mbx.UserPrincipalName
    
    # Compare size to warning quota (values are in bytes)
    if ($stats.TotalItemSize.Value -ge $mbx.IssueWarningQuota.Value) {
        $report += [PSCustomObject]@{
            DisplayName = $mbx.DisplayName
            EmailAddress = $mbx.PrimarySmtpAddress
            CurrentSize_GB = [math]::Round($stats.TotalItemSize.ToGB(), 2)
            WarningQuota_GB = [math]::Round($mbx.IssueWarningQuota.ToGB(), 2)
        }
    }
}

$report | Export-Csv -Path "C:\Reports\UsersOverWarningQuota.csv" -NoTypeInformation -Encoding UTF8
                            

Note: Instead of writing this complex PowerShell script and running it manually, you can use the Current Mailbox Size vs Quota report in M365 Manager Plus to get a list of mailbox quota exceeded users. If you wish to get the mailbox quota warning message for Microsoft 365 mailboxes periodically, you can also schedule the reports to get a weekly report of mailbox quota exceeded users, all without a single PowerShell script.

Method 3: How to export Microsoft 365 mailbox quotas in M365 Manager Plus

1. Log in to M365 Manager Plus and click the Reports tab.
2. Navigate to Exchange Online > Mailbox Reports in the left pane.
3. Click the Mailbox Size Restrictions report. You can now view a list of all mailbox quota details of your Microsoft 365 mailboxes, such as prohibit send quota, prohibit send receive quota, issue warning quota, archive quota, and more.
4. Click Export As and select your desired file format (CSV, XLSX, PDF, or HTML) to export the list of Microsoft 365 mailbox size quotas.

   

Get mailbox quota warning messages for Microsoft 365 mailboxes with M365 Manager Plus

To get the mailbox quota warning message for Microsoft 365 mailboxes that exceed their storage limit using M365 Manager Plus, you can create a custom report by filtering the Current Mailbox Size vs Quota report to identify users who have surpassed a specific percentage of their Prohibit Send and Receive quota.

Follow these steps to get your weekly mailbox exceeded users report:

1. Log in to M365 Manager Plus, navigate to Reports, and select Schedule Report.
2. Click Create Schedule.
3. Type in a Scheduler Name and select the tenant for which you want to generate and schedule this report.
4. Select Exchange Online in the Microsoft 365 Service drop-down and select the Current Mailbox Size vs Quota report.
5. Hover over the report under Selected Reports and click the edit icon, then click More Options to filter the report further.
6. Click Add Filter, select Quota Used (%) in the first drop-down, Greater than or equals in the second, and type in your preferred threshold percent in the field next to it.
7. Click Apply.
8. Configure the schedule using the Scheduled to Run drop-downs.
9. Select the format in which you want the report to be downloaded or emailed using the Export As drop-down. Configure the Storage Path to download the reports locally.
10. In the Notification Settings drop-down, select Notify all reports.
11. If you want the reports emailed to you, use the Select Notification Template to select a format for your emails. This option will be accessible once you configure your email server.
12. Click Save to schedule mailbox quota warning messages for your Microsoft 365 mailboxes.

Note: You can also create this filter as a custom report in M365 Manager Plus if you wish to generate it whenever you want to check for mailbox quota exceeded users.

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

$users = Get-EXOMailbox -ResultSize Unlimited -Filter "Department -eq 'Department'"

foreach ($user in $users) {
    Set-Mailbox -Identity $user.UserPrincipalName `
        -IssueWarningQuota 90GB `
        -ProhibitSendQuota 95GB `
        -ProhibitSendReceiveQuota 100GB
}