As an Exchange admin, you may get requests from users to increase mailbox size in Office 365 due to the ‘Mailbox size full’ warning. As you know, mailbox storage size is determined by the type of mailbox and the license assigned to them. So, to increase mailbox size, you must assign them a different subscription plan.  

Mailbox Storage Limit:  

You can refer to the below image to know the storage limit based on the mailbox type and license. 

Shared mailboxes do not require licenses until their mailbox size exceeds 50 GB. To increase the mailbox size from 50 GB to 100 GB, you must assign an Exchange Online Plan 2 license.  

  

How to Check Mailbox Usage Size in Exchange Online?  

To get mailbox size, you can use the ‘Get-MailboxStatistics’ cmdlet as below. To use Exchange Online PowerShell cmdlets, you must connect to Exchange Online PowerShell.  

Get-MailboxStatistics –Identity <UPN> | Select TotalItemSize  

You can also download the pre-built script to generate a mailbox size report for all the mailboxes. By referring to this report, admins can set mailbox size quota based on the requirement.  

Sample Output:

How to Check Mailbox Quota in Microsoft 365?  

Based on the mailbox requirement, admins can customize a mailbox quota. Generally, mailbox quota is determined by the following 3 attributes.  

• IssueWarningQuota – When hit the limit, outlook will show a ‘Mailbox is almost full’ warning.  
• ProhibitSendQuota – When the mailbox reaches the limit, it can’t send mail anymore. 
• ProhibitSendReceiveQuota – When mailbox size reaches the limit, mailboxes can’t send and receive emails.  

You can run the ‘Get-Mailbox’ cmdlet to view the current mailbox quota. 

Get-Mailbox –Identity <UPN> | Select *quota  

  

How to Increase Mailbox Size in Exchange Online using PowerShell?  

Admins can use the ‘Set-Mailbox’ cmdlet to increase Office 365 mailbox size.  

Set-Mailbox –Identity <UPN> - IssueWarningQuota <Size> -ProhibitSendQuota <size> -ProhibitSendReceiveQuota <Size>  

  

To set a mailbox quota for all the mailboxes, you need to combine both ‘Get-Mailbox‘ and ‘Set-Mailbox‘ cmdlets. For example,  

Get-Mailbox | Set-Mailbox - IssueWarningQuota <Size> -ProhibitSendQuota <size> -ProhibitSendReceiveQuota <Size>  

  

How to Increase Mailbox Size without Changing License Plan?  

If the mailbox’s existing subscription supports archiving, you can increase the mailbox size without changing the subscription plan. By enabling archive mailboxes, users can get up to 100 GB of additional storage.  

 If users need more storage space, admins can enable auto-expanding archiving to provide up to 1.5 TB of additional storage in archive mailboxes. You can check the archive mailbox size using PowerShell.  

  

I hope this blog will help admins customize mailbox size using PowerShell. How do you manage mailbox size in your organization? You can share your techniques and ideas with fellow admins and us through the comment section.