Mailbox storage, archiving and retention

When a mailbox fills up, and what keeping mail actually means

Who this is for: Administrators with Global Administrator rights or membership in the Exchange Online Organization Management role group. The organization-wide auto-expanding archiving task requires one of these roles.

Exchange admin center - Recipients > Mailboxes. Open a mailbox to see its size and quota.

A mailbox is full

Scope: This guide covers organization-wide auto-expanding archiving and the storage and licensing facts in the supplied sources. It does not provide mailbox-size inspection or cleanup steps, instructions to enable a primary archive, per-user auto-expanding setup, or retention-policy setup. Auto-expanding archiving adds storage to an archive mailbox; it does not increase the primary mailbox. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving

Shared mailbox storage limits

The standard limit for an unlicensed shared mailbox is 50 GB. Unlicensed shared mailboxes created before July 2018 have a 100 GB limit. At its limit, a shared mailbox might receive email for a time but cannot send new email; later it can stop receiving email, and senders can receive a non-delivery receipt. To raise the standard limit to 100 GB, assign the shared mailbox an Exchange Online Plan 2 license. Mailboxes covered by the pre-July 2018 exception already have a 100 GB limit. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes

Note A shared mailbox with in-place archiving and increased archive storage requires Exchange Online Plan 2, or Exchange Online Plan 1 with the Exchange Online Archiving add-on. A user accessing a shared mailbox must have a licensed Exchange Online mailbox. If you need to assign a license, see guide 5, Manage Microsoft 365 licenses. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes

Auto-expanding archiving

Auto-expanding archiving adds storage automatically to an archive mailbox as needed. The standard maximum is 1.5 TB. The 3 TB maximum applies to mailboxes with eligible licenses: Office 365 E5, A5, or G5; Microsoft 365 E5, A5, or G5; Microsoft 365 Purview Suite; Microsoft 365 Purview Suite FLW, EDU, or GOV; Microsoft Defender + Purview Suite FLW; Microsoft 365 F5 Compliance; Microsoft 365 F5 Security + Compliance; the combination of the eDiscovery & Audit, Insider Risk Management, and Information Protection & Governance mini suites; and qualifying offerings that include Microsoft Purview E5 capabilities, such as E7. Other licenses remain subject to the 1.5 TB limit. For E5 licenses, Records Management must be selected and remain enabled in the user's license settings. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving

Before you start

The archive mailbox for each user mailbox that needs archive storage must already be enabled. For new user mailboxes, enable the main archive as well. This guide does not include the steps to enable a main archive. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
To enable the feature for the whole organization, the administrator must be a Global Administrator or a member of the Organization Management role group in Exchange Online. The supplied source specifies no separate administrator license requirement for this action. If the required role is missing, have an administrator with one of these roles perform the task. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
The source does not specify a separate license required just to enable organization-wide auto-expanding archiving. The license determines whether the mailbox has a 1.5 TB or eligible 3 TB maximum. For E5 licenses, ensure Records Management is selected and enabled. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
Auto-expanding archiving also supports shared mailboxes. For a shared mailbox with in-place archiving, assign Exchange Online Plan 2 or Exchange Online Plan 1 with the Exchange Online Archiving add-on. If that license is missing, stop and assign it before relying on the shared mailbox's archive storage. For license assignment steps, see guide 5, Manage Microsoft 365 licenses. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes

Important Do not enable auto-expanding archiving until you have reviewed the consequences. Once enabled for the organization, it cannot be turned off. It can also prevent recovery or restoration of a mailbox that later becomes inactive. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving

Enable it for the organization

Connect to Exchange Online PowerShell using an account with the Global Administrator role or membership in the Exchange Online Organization Management role group. If you need instructions for establishing an Exchange Online PowerShell session, see guide 66, Manage Exchange Online with PowerShell. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
In the connected Exchange Online PowerShell session, run the organization-wide command below. It enables auto-expanding archiving for existing and new user mailboxes. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
Verify the organization setting with the command below. Confirm that the output shows AutoExpandingArchiveEnabled as True. If it does not, check that the command ran in the correct Exchange Online organization and that the connected account has the required role, then run the enablement command again. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving

Set-OrganizationConfig -AutoExpandingArchive

Get-OrganizationConfig | FL AutoExpandingArchiveEnabled

Note The organization-wide setting does not enable a user's main archive. When creating a new user mailbox, enable that user's main archive so auto-expanding archiving can work for it. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving

After the archive mailbox, including its Recoverable Items folder, reaches 90 GB, it is converted to an auto-expanding archive. Additional storage can take up to 30 days to be provisioned. Keep the license eligibility in place and monitor archive growth during that time. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving

Retention - keeping and deleting on purpose

Retention policies are an advanced feature, and licenses might be required. This guide does not give steps to create or apply a retention policy. For mailbox litigation hold and retention procedures, see guide 84, Litigation hold and retention for mailboxes. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes
A shared mailbox placed on litigation hold requires Exchange Online Plan 2, or Exchange Online Plan 1 with the Exchange Online Archiving add-on. If that license is missing, assign the required license before placing the shared mailbox on hold. For license assignment steps, see guide 5, Manage Microsoft 365 licenses. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes

Important Do not use this guide to choose or apply a retention or deletion policy. The supplied sources do not provide policy configuration steps. Review the intended policy and licensing requirements in guide 84, Litigation hold and retention for mailboxes, before making retention changes. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes

When it goes wrong

If a mailbox does not have its main archive enabled, stop: an archive must already be enabled before auto-expanding archiving can be enabled for that mailbox. Enable the main archive using your organization's archive-provisioning procedure, then return to the auto-expanding step. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
If you cannot run the organization-wide command because your account lacks the required role, have a Global Administrator or a member of the Exchange Online Organization Management role group run it. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
If the primary mailbox is on-premises and the archive mailbox is cloud-based in an Exchange hybrid deployment, do not use a per-user Enable-Mailbox -AutoExpandingArchive command. The supported method for this hybrid case is the organization-wide Set-OrganizationConfig -AutoExpandingArchive command in Exchange Online PowerShell. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
If the organization setting verifies as enabled but an archive has not yet received more storage, allow up to 30 days after the archive, including Recoverable Items, reaches 90 GB for provisioning. Continue monitoring archive growth. Source: https://learn.microsoft.com/en-us/purview/enable-autoexpanding-archiving
If the mailbox is a shared mailbox at its 50 GB limit, remember that it might stop sending first and later stop receiving. Assign Exchange Online Plan 2 to raise its limit to 100 GB; in-place archive storage instead requires Exchange Online Plan 2 or Exchange Online Plan 1 with the Exchange Online Archiving add-on. Source: https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes