An archive mailbox starts at 100 GB, and for heavy mail users that eventually runs out. Auto-expanding archiving lets Exchange Online add storage on its own, but it comes with licensing rules, a switch you can't turn back off and some slow timelines. In this post I'll cover what to check first, how to enable it, how items actually reach the archive, and how to tell whether expansion has happened.
Prerequisites#
- Licensing. Exchange Online Plan 2, Microsoft 365 E3 and E5, Office 365 E3 and E5, and Microsoft 365 Business Premium already include it. Plans such as Exchange Online Plan 1, Microsoft 365 Business Basic and Business Standard need the Exchange Online Archiving add-on. Shared mailboxes are supported, but an archive may only be used for a licensed user or entity, so license the shared mailboxes that need one.
- An enabled archive. Auto-expanding builds on the main archive; it doesn't create one.
- Permissions. Global Administrator or the Organization Management role group for the org-wide switch; a role group with the Mail Recipients role is enough for individual mailboxes.
- Exchange Online PowerShell. There's no toggle for this in the Exchange admin center or the Microsoft Purview portal.
Watch out: once enabled for the organization or for a mailbox, auto-expanding archiving can't be turned off. It also limits inactive mailboxes: if an auto-expanding mailbox later becomes inactive, you can't recover or restore it, only export its content with Content search. In hybrid deployments, a cloud archive with auto-expanding enabled can't be moved back on-premises.
Know the limits too. The archive grows to a maximum of 1.5 TB, Recoverable Items included. Growth of up to 1 GB per day is supported, and using journaling, transport rules or forwarding to fill an archive isn't permitted. Microsoft has also announced a 3 TB ceiling for mailboxes with eligible E5-level licenses, scheduled to roll out from mid-October 2026; other licenses stay at 1.5 TB.
Step 1: Check and enable the main archive#
Connect-ExchangeOnline
Get-Mailbox -Identity megan@contoso.com | Format-List ArchiveStatus, AutoExpandingArchiveEnabled, RetentionPolicy
Enable-Mailbox -Identity megan@contoso.com -ArchiveSkip the second command if ArchiveStatus already shows Active.
Step 2: Turn on auto-expanding archiving#
Option A: the whole organization#
Set-OrganizationConfig -AutoExpandingArchive
Get-OrganizationConfig | Format-List AutoExpandingArchiveEnabledThis covers existing and new mailboxes, as long as each has its main archive enabled. Two things surprise people: it doesn't set AutoExpandingArchiveEnabled to True on individual mailboxes, and it doesn't add the 10 GB buffer described below. Where primary mailboxes are on-premises and archives are in the cloud, this is the only way to enable it.
Option B: specific mailboxes#
Enable-Mailbox -Identity megan@contoso.com -AutoExpandingArchive
Get-Mailbox -Identity megan@contoso.com | Format-List AutoExpandingArchiveEnabledIf the mailbox is on hold or has a retention policy, this also raises the archive quota from 100 GB to 110 GB (warning at 100 GB) and does the same for Recoverable Items, which bridges the gap until extra storage arrives. Already enabled org-wide but want that buffer for one user? Run the command anyway: you'll get an "already enabled" error, but the space is still added.
Step 3: Make sure items actually move#
Auto-expanding archiving only adds space. Something still has to move items out of the primary mailbox: an Exchange retention policy with Move to archive tags, processed by the Managed Folder Assistant. The built-in Default MRM Policy includes a tag that moves items after two years. If that's too slow for a busy mailbox, create your own:
New-RetentionPolicyTag -Name "Archive after 1 year" -Type All -RetentionEnabled $true `
-AgeLimitForRetention 365 -RetentionAction MoveToArchive
New-RetentionPolicy -Name "Archive after 1 year" -RetentionPolicyTagLinks "Archive after 1 year"
Set-Mailbox -Identity megan@contoso.com -RetentionPolicy "Archive after 1 year"
# Ask the Managed Folder Assistant to process this mailbox
Start-ManagedFolderAssistant -Identity megan@contoso.comA mailbox can have only one MRM retention policy at a time, so add any other tags you rely on (deleted items, personal tags) to the new policy. The assistant visits every mailbox at least once every seven days on its own; Start-ManagedFolderAssistant only asks it to get to this one sooner, so allow some time.
Verify: has the archive expanded?#
An expanded archive consists of the main archive plus one or more auxiliary archives. This lists every location with its size:
Get-MailboxLocation -User megan@contoso.com | ForEach-Object {
$stats = Get-MailboxStatistics -Identity $_.MailboxGuid.ToString()
[pscustomobject]@{
Location = $_.MailboxLocationType
ItemSize = $stats.TotalItemSize
DeletedItems = $stats.TotalDeletedItemSize
}
}If there's no AuxArchive row, no extra storage has been provisioned yet; each AuxArchive row is additional storage. Look at the deleted items figure too, because Recoverable Items counts towards the archive's size and its limit.
Tips & gotchas#
- Archive nearly full, no expansion yet: expansion starts when the archive, including Recoverable Items, approaches its quota (Microsoft cites 90 GB), and provisioning can take up to 30 days. Enable it well before a mailbox hits the wall.
- Primary mailbox still filling up: check the mailbox's
RetentionPolicy, and make sureRetentionHoldEnabledandElcProcessingDisabledaren'tTrue, since either stops items from moving. - Unfamiliar folders with names like
Inbox_2021 (Created on ...)are the service redistributing content into auxiliary storage. Warn users before it happens. - User experience changes: once an auxiliary archive exists, users can't delete archive folders, Recover Deleted Items doesn't work for items deleted from the expanded area, and a classic Outlook search from the primary mailbox won't return auxiliary archive items. Item and unread counts may also be inaccurate.
- At the ceiling, nothing more moves into the archive, the primary mailbox can fill up, and sending and receiving can stop. Use retention policies to delete content that no longer has business value.