How to convert a user mailbox to a shared mailbox

Converting a user mailbox to a shared mailbox in Exchange Online is one command or one button, and it keeps all the mail, calendar, and contacts while letting you drop the licence — provided the mailbox is under 50 GB and not on hold. It is the standard move when a leaver's inbox needs to stay readable by a successor, and the standard mistake is removing the licence before the conversion has finished.

How shared mailboxes behave, and when a Microsoft 365 group or distribution list is the better shape, is covered in shared mailboxes explained and migrating between shared mailboxes and distribution groups. This page is the conversion itself, usually as a step in the offboarding process.

Prerequisites

• Exchange Administrator (or Recipient Administrator) role.
• Exchange Online PowerShell module for the scripted route.
• The mailbox is under 50 GB (check with Get-MailboxStatistics), or you accept it will still need a licence.
• The mailbox is not on litigation hold or a retention hold you cannot remove, or you accept it will still need a licence. Check with Get-Mailbox | fl LitigationHoldEnabled,InPlaceHolds.
• For synced (hybrid) users: the user object stays in on-premises AD; the conversion is done in Exchange Online and works, but the msExchRemoteRecipientType attribute on-premises will not reflect it unless you update it — see the hybrid deployment guide. Plan to also disable the AD account and block sign-in.

Steps

1. Check size and holds

Connect-ExchangeOnline
Get-MailboxStatistics leaver@contoso.com | Select-Object TotalItemSize
Get-Mailbox leaver@contoso.com | Format-List LitigationHoldEnabled,InPlaceHolds,ArchiveStatus,RecipientTypeDetails

If a hold applies and legal wants it kept, stop here and keep the licence; conversion still works but changes nothing about licensing.

2. Block sign-in and clear the password

If this is a leaver, block sign-in in the Microsoft 365 admin center (or Entra) and revoke sessions before converting. A shared mailbox has a disabled account by design, but a converted mailbox keeps whatever sign-in state the user had, so make it explicit. The full leaver sequence is in how to offboard a user.

3. Convert

Exchange admin center: Recipients → Mailboxes → the mailbox → Others → Convert to shared mailbox. Or:

Set-Mailbox leaver@contoso.com -Type Shared

Conversion is usually complete within a minute; Get-Mailbox leaver@contoso.com | Select RecipientTypeDetails reports SharedMailbox when done.

4. Grant access

Add-MailboxPermission leaver@contoso.com -User successor@contoso.com -AccessRights FullAccess -AutoMapping $true
Add-RecipientPermission leaver@contoso.com -Trustee successor@contoso.com -AccessRights SendAs -Confirm:$false

Full Access with auto-mapping makes the mailbox appear in the successor's Outlook automatically. Use Send on Behalf (Set-Mailbox -GrantSendOnBehalfTo) instead of Send As if replies should visibly come from the successor.

5. Set an auto-reply, then remove the licence

Set an out-of-office that names the new contact (Set-MailboxAutoReplyConfiguration -AutoReplyState Enabled -ExternalMessage ... -InternalMessage ...). Then, and only then, remove the licence in the admin center or via group-based licensing. The mailbox stays; it is now unlicensed shared.

Verify

• Get-Mailbox shows SharedMailbox, and the admin center's Shared mailboxes list includes it.
• The successor sees the mailbox in Outlook (allow up to an hour for auto-mapping) and can send a test message as/on behalf.
• 24 hours after licence removal, the mailbox is still present and receiving mail; the licence count dropped by one.

Roll back

Set-Mailbox leaver@contoso.com -Type Regular converts it back, but a regular mailbox needs a licence within the 30-day grace period or it is soft-deleted. Assign the licence first, then convert. If the licence was removed prematurely and the mailbox has gone soft-deleted, restore it from Users → Deleted users (Entra) within 30 days; see the group restore runbook for the equivalent group-mailbox case.