Exchange Online Mailbox Size Limits: 50 GB, 100 GB and Archive Explained

Exchange Online gives a user mailbox 50 GB on Exchange Online Plan 1 and Office 365 E1, 100 GB on Exchange Online Plan 2, Microsoft 365 Business Basic, Standard and Premium and the E3 and E5 plans, and 2 GB on Exchange Online Kiosk and Office 365 F3. If that is not enough, you enable the archive mailbox for up to 100 GB more, and on eligible licences turn on auto-expanding archiving, which grows the archive up to 1.5 TB.

Who this is for and what you will have at the end

This guide is for Microsoft 365 administrators who get "mailbox full" tickets, plan licences, or need to decide between a larger plan, an archive and a cleanup policy.

At the end you will have:

• The size limit that applies to each mailbox type and licence.
• The PowerShell to check how full primary and archive mailboxes are.
• The steps to enable archives and auto-expanding archives, with their restrictions.
• An understanding of what the default policy moves and when.

Mailbox size limits by licence

Microsoft's Exchange Online limits page states that the mailbox type and the user's subscription licence determine the storage available, and that administrators can reduce maximum mailbox sizes per user or globally.

Microsoft 365 and Office 365 plans

Where the table shows a 1.5 TB archive, Microsoft's footnote explains that each user initially receives 100 GB of storage in the archive mailbox, and that additional storage is added only when auto-expanding archiving is turned on.

Standalone Exchange Online plans

Archive mailboxes are not included in Exchange Online Kiosk. They can be bought as an add-on through Exchange Online Archiving, and an Exchange Online Plan 1 licence with the Exchange Online Archiving add-on also supports auto-expanding archiving up to 1.5 TB.

Notes that catch administrators out

• A shared mailbox does not need its own licence, but without one it is limited to 50 GB. To raise it to 100 GB, the mailbox needs a plan that allows it, such as Exchange Online Plan 2, and a user accessing it needs an Exchange Online licence.
• Placing a shared mailbox on litigation hold needs an Exchange Online Plan 2 licence, or Plan 1 with Exchange Online Archiving. Features such as Microsoft Defender for Office 365, eDiscovery (Premium) and retention policies require the shared mailbox to be licensed for them.
• Resource mailboxes (rooms and equipment) do not need a licence, and are limited to 50 GB without one.
• Public folder mailboxes in Exchange Online: you are limited to 1,000 public folder mailboxes. The default individual public folder size limit is 2 GB.
• Archive mailboxes may only archive mail for a single user or entity for which a licence has been applied. Copying mail from many users to one mailbox through journaling, transport rules or auto-forwarding to archive it is not permitted.

Recoverable Items quotas

Deleted items go to the Recoverable Items folder, which has its own quota. In the primary mailbox it is 30 GB, or 100 GB when the mailbox is on hold. When a mailbox is placed on hold and archiving is enabled, quotas are adjusted: for a mailbox on hold or assigned a retention policy, enabling the archive raises the Recoverable Items quota from 100 GB to 105 GB and the warning quota from 90 GB to 95 GB. For how holds relate to retention, see litigation hold vs retention policy in Exchange.

Step 1: Check how full mailboxes are

Connect to Exchange Online PowerShell, then check the primary mailbox:

Get-MailboxStatistics -Identity [email protected] | Format-List DisplayName,TotalItemSize,ItemCount

Microsoft recommends Get-EXOMailboxStatistics instead in Exchange Online PowerShell. Use the -Archive switch to see the archive mailbox that belongs to the user:

Get-MailboxStatistics -Identity [email protected] -Archive | Format-List DisplayName,TotalItemSize,ItemCount

Compare the primary mailbox size with the limit for the user's licence in the tables above. The archive size matters as much as the primary size, because the primary can only offload to the archive while the archive has space.

Step 2: Enable the archive mailbox

An archive mailbox, also called In-Place Archiving, is an additional mailbox associated with the user's primary mailbox. Users can reach it in Outlook and Outlook on the web.

You need the Mail Recipients role in Exchange Online to enable archives. By default it is held by the Recipient Management and Organization Management role groups.

In the Exchange admin center

1. Open the Exchange admin center and go to Recipients > Mailboxes.
2. Select the user.
3. In the flyout, select Others, and under Mailbox archive select Manage mailbox archive.
4. Turn on Mailbox archive and select Save.

The Archive status column shows Active when it has been created, after a refresh.

With PowerShell

For one user:

Enable-Mailbox -Identity [email protected] -Archive

For all user mailboxes that do not have an archive and have not had one disabled:

Get-Mailbox -Filter {ArchiveGuid -Eq "00000000-0000-0000-0000-000000000000" -AND DisabledArchiveGuid -Eq "00000000-0000-0000-0000-000000000000" -AND RecipientTypeDetails -Eq "UserMailbox"} | Enable-Mailbox -Archive

If an archive was disabled more than 30 days ago, the command fails with a MissingDisconnectReceiptsException error. Microsoft's fix is to run Set-Mailbox for that mailbox with -RemoveDisabledArchive, then run the enable command again.

Auto-enable on a full mailbox

You can have Exchange Online provision an archive automatically when a primary mailbox reaches 90% of its quota:

Set-OrganizationConfig -AutoEnableArchiveMailbox $true

An archive is created automatically only if the mailbox is a user or shared mailbox in Exchange Online, archiving is included in the assigned licence, and the mailbox is not synchronized from on-premises.

What actually moves

When you enable the archive, items older than the archive policy in the assigned retention policy are moved to the archive. The default archive policy in the default retention policy that Exchange Online assigns moves items two years after they were delivered or created. It also moves items 14 days or older from the primary Recoverable Items folder to the archive's Recoverable Items folder.

Note that, if you have created a retention policy in Exchange Online, messages are moved automatically only if the primary mailbox is larger than 10 MB.

If you disable an archive, items stay in the primary mailbox, and you can reconnect the archive within 30 days with its contents restored. After 30 days the contents are permanently deleted.

Step 3: Enable auto-expanding archiving

Without auto-expanding archiving the archive is capped at its initial quota. With it, Microsoft Purview periodically checks the archive size and adds storage until the archive reaches 1.5 TB.

Know the restrictions first

• It cannot be turned off after you enable it, for the organization or for a user.
• Administrators cannot adjust the quota.
• It prevents recovering or restoring an inactive mailbox.
• The archive must already be enabled.
• You cannot use the Exchange admin center or the Purview portal. You must use Exchange Online PowerShell.
• It is supported for mailboxes used by individual users or shared mailboxes with a growth rate that does not exceed 1 GB per day.
• After you enable it, the archive converts to an auto-expanding archive when it reaches 90 GB including Recoverable Items, and the extra storage can take up to 30 days to be provisioned.
• After an auto-expanded storage area exists, users cannot delete folders in the archive, and cannot use Recover Deleted Items for items in it.

Turn it on

For everyone:

Set-OrganizationConfig -AutoExpandingArchive

For one user (the archive must already be enabled):

Enable-Mailbox [email protected] -AutoExpandingArchive

When you enable it for one user whose mailbox is on hold or has a retention policy, the archive quota rises from 100 GB to 110 GB, and the warning quota from 90 GB to 100 GB, to bridge the gap until the extra storage is provisioned. The organization-wide command does not add that extra space.

Verify:

Get-OrganizationConfig | FL AutoExpandingArchiveEnabled
Get-Mailbox [email protected] | FL AutoExpandingArchiveEnabled

A value of True means it is enabled. Setting it for the whole organization does not change the property on individual mailboxes.

Larger limit for E5 licences

Microsoft states that it is raising the maximum auto-expanding archive capacity for eligible mailboxes from 1.5 TB to 3 TB, with rollout beginning in mid-October 2026 and expected to complete by late October. It applies to licences such as Office 365 E5, A5 and G5, Microsoft 365 E5, A5 and G5, and the Microsoft 365 Purview Suite. For all other licences the 1.5 TB limit stays. For E5 licences, Records Management must be selected in the user's licence settings. Check the documentation again before you rely on the 3 TB figure, because rollout dates can move.

Step 4: Know about auto-archiving

Auto-archiving is a separate, threshold-based feature. When a primary mailbox exceeds 96% of its quota by default, and the archive is provisioned and has space, the Managed Folder Assistant moves the oldest items first. Items tagged Never Move to Archive, and items received less than 60 days ago, are excluded.

Microsoft's page describes the feature as being in preview with availability on the Microsoft 365 Roadmap, so confirm it is available in your tenant. It does not create archives or turn on auto-expanding archiving, and if the main archive is full without auto-expanding archiving, it moves nothing. The threshold is set organization-wide with Set-OrganizationConfig -AutoArchivingThresholdPercentage, with a valid range of 80 to 100, and 100 turns the feature off for the tenant. A user can be excluded with Set-Mailbox <user> -AutoArchivingEnabled $false.

Troubleshooting

Summary checklist

• Confirm the licence of the mailbox and look up its limit in the tables.
• Check primary and archive sizes with Get-MailboxStatistics, using -Archive for the archive.
• Enable the archive, and decide whether you want automatic provisioning at 90%.
• Enable auto-expanding archiving only where users need it, because it cannot be reversed.
• Use retention policies to delete content that has no business value, so mailboxes do not grow without bound.
• Review shared mailboxes and give licences where they need more than 50 GB or holds.

References

• Exchange Online limits (Microsoft Learn)
• Enable archive mailboxes (Microsoft Learn)
• Learn about archive mailboxes (Microsoft Learn)
• Learn about auto-expanding archiving (Microsoft Learn)
• Enable auto-expanding archiving (Microsoft Learn)
• Auto-archiving in Exchange Online (Microsoft Learn)
• Get-MailboxStatistics (Microsoft Learn)