To enable or configure a mailbox quota in Microsoft 365 (Exchange Online), you typically use Exchange Online PowerShell because the modern Microsoft 365 admin center doesn’t provide full control over quota settings for most mailbox types.
🛠️ Prerequisites
You need to be assigned the Exchange administrator role.
Install and connect to Exchange Online PowerShell:
Connect-ExchangeOnline -UserPrincipalName youradmin@yourdomain.com
📏 Default Mailbox Quotas
By default:
Exchange Online Plan 1: 50 GB mailbox size
Exchange Online Plan 2 / E3/E5: 100 GB mailbox size
To customize quota warnings or limits:
⚙️ Set Mailbox Quota via PowerShell
Set-Mailbox user@yourdomain.com ` -IssueWarningQuota 49.5GB ` -ProhibitSendQuota 49.9GB ` -ProhibitSendReceiveQuota 50GB
Replace user@yourdomain.com with the actual user's email address.
Parameters:
IssueWarningQuota: Sends a warning email when the mailbox reaches this size.ProhibitSendQuota: Prevents the user from sending emails once this limit is hit.ProhibitSendReceiveQuota: Prevents sending/receiving once this limit is reached.
🔍 Check Current Quota Settings
Get-Mailbox user@yourdomain.com | Format-List IssueWarningQuota,ProhibitSendQuota,ProhibitSendReceiveQuota
🚫 Notes
You cannot increase a mailbox beyond 100 GB with standard plans. For larger storage, consider archive mailboxes with auto-expanding archiving.
Some settings may be inherited from mailbox plans, so you might need to override those explicitly.
NOTE: Liongard uses the following metric Users[?mailQuotaExceeded_r == `true`] | length(@) which provides information in "True" and "False" values. The threshold of a mailbox is set in Microsoft, and the above-mentioned Powershell commands can be used for checking the threshold as well as setting a new limit.