Office 365 – Converting mailboxes to shared mailboxes

Office 365 introduces the concept of a shared mailbox. Shared mailboxes are free, as long as they are under 5GB in size.

There is plenty of documentation that describes the process to create a new shared mailbox, but how do you convert an existing licensed mailbox to a shared mailbox?

I have read through a number of Office 365 community forum threads that ask this question and provide a variety of methods to achieve this. These methods include backing up data/recreating the mailbox/restoring the data or adding/removing SMTP addresses across 2 mailboxes. These methods are not suitable if you have a large number of mailboxes to convert, or if you’re lazy.

Now I cannot say whether the following is a ‘Microsoft supported’ approach, but it works and I have used it to convert over 500 mailboxes to shared mailboxes. Once you have an Office 365 PowerShell session established, the conversion involves 3 steps:

1. Confirm combined TotalDeletedItemSize and TotalItemSize is under the size limit for a shared mailbox. The shared mailbox size limit is 5GB, but we’re going to be setting warning levels at 4.5GB so ideally you want the mailbox to be less than 4GB.

Get-MailboxStatisics <Alias> | FL Total*

2. Configure the shared mailbox parameters and assign any required permissions:

Set-Mailbox -Identity <Alias> -Type “Shared” -ProhibitSendReceiveQuota 5GB -ProhibitSendQuota 4.75GB -IssueWarningQuota 4.5GB

Add-MailboxPermission -Identity <Shared Mailbox Alias> -User <User/Group Alias> -AccessRights FullAccess -InheritanceType All

Add-RecipientPermission -Identity <Shared Mailbox Alias> -Trustee <User/Group Alias> -AccessRights SendAs -Confirm:$false

3. Revoke the license from the Office 365 Administration Console, or via PowerShell:

$MSOLSKU = (Get-MSOLUser -UserPrincipalName <UPN>).Licenses[0].AccountSkuId
Set-MsolUserLicense -UserPrincipalName <UPN> -RemoveLicenses $MSOLSKU

If you want to complete this process for a large number of mailboxes, the following script is an example of how you might do this. This script assumes you have already confirmed the mailboxes are under shared mailbox size limit. The script comes without warranty of any kind so please use at your own risk.

You will need an input file C:\temp\input.csv containing the userPrincipalNames for the mailboxes you wish to convert. e.g.

userPrincipalName

user1@domain.com.au

user2@domain.com.au

user3@domain.com.au

Now execute the script in an Office 365 PowerShell session:

Import-csv C:\temp\input.csv | foreach {
$UPN = $_.userPrincipalName
Set-Mailbox $UPN -Type “Shared” -ProhibitSendReceiveQuota 5GB -ProhibitSendQuota 4.75GB -IssueWarningQuota 4.5GB
$MSOLSKU = (Get-MSOLUser -UserPrincipalName $UPN).Licenses[0].AccountSkuId
Set-MsolUserLicense -UserPrincipalName $UPN -RemoveLicenses $MSOLSKU
}