Microsoft Will Finally Retire Site Mailboxes in April 2021

Time to Kill Site Mailboxes

Office 365 notification MC224531 (October 19) came as a surprise because who could have known that site mailboxes are still around? After all, in January 2017, Microsoft announced their intention to block the creation of new site mailboxes. Nearly four years later, you might think that organizations had figured out how to dump these unfortunate creations.

The History of Site Mailboxes

Site mailboxes came about in a Microsoft effort to show that Exchange 2013 and SharePoint 2013 could work together to deliver value. In on-premises environments, the long checklist of steps needed to bring the two servers together put many off.

Figure 1: Site Mailboxes (image credit: Microsoft)

Things were much easier inside Office 365 where Microsoft took care of the configuration. Even so, site mailboxes were obsoleted by developments soon after they were released with the advent of Office 365 Groups (now Microsoft 365 Groups). Since Office 365 Groups came along, we’ve seen an increasing amount of integration across the Microsoft 365 ecosystem encouraged by the availability of a common platform (the substrate) and common Graph APIs.

In any case, the final curtain for site mailboxes descends in April 2021. Microsoft says that they will retire site mailboxes at that point.

If Your Tenant Has Site Mailboxes

If you didn’t receive the notification, Microsoft hasn’t found any site mailboxes in your tenant and you have nothing to worry about. Following Microsoft’s advice, I looked to see what site mailboxes existed and found three were present:

# Find details of site mailboxes known to the tenant
Get-SiteMailbox -BypassOwnerCheck -ResultSize Unlimited | Format-List Name, WhenCreated, SharePointUrl, Owners

Name          : SMO-Office365forExchangeProfessionals
WhenCreated   : 02/01/2015 10:33:33
SharePointUrl :
Owners        : {TRedmond, Ben Owens, Kim Akers}

Name          : O365-ExchangeConnections2015
WhenCreated   : 16/02/2015 11:22:14
SharePointUrl : https:// Connections 2015
Owners        : {TRedmond}

Name          : SMO-Projects
WhenCreated   : 27/01/2014 20:36:50
SharePointUrl : https://
Owners        : {TRedmond}

All the site mailboxes date from 2014-2015, right around the time when we first started working on the Office 365 for IT Pros book (the first two versions were called Office 365 for Exchange Professionals). Microsoft launched Office 365 Groups in November 2014 and we switched to Groups soon afterwards.

No trace of the SharePoint sites could be found. They might have been removed in the past during a purge of inactive sites. If the sites were available, their content could be retrieved using Office 365 content searches and exported (Microsoft has a script to do the job on GitHub).

Failure to Clean Up

Site mailboxes don’t really contain any real items. Instead, they hold stubs pointing to documents in SharePoint libraries. The stubs are small, so it’s easy to detect folders used for this purpose because they usually have many items but are quite small. A check of the site mailboxes with Get-ExoMailboxFolderStatistics revealed that there was nothing much in any mailbox:

# Get details of what's in site mailboxes
Get-ExoMailbox -RecipientTypeDetails TeamMailbox | Get-ExoMailboxFolderStatistics | Format-Table Name, ItemsInFolder

Site mailboxes have a recipient type of TeamMailbox because at one time that’s what they were going to be called. Luckily Microsoft changed the name to focus on sites as otherwise they would have had to find a new name for Teams.

In the knowledge that the site mailboxes were useless artefacts of the past, I tried to delete them. Unhappily, the attempt failed with some horrible errors because the linked SharePoint sites couldn’t be found:

Get-ExoMailbox -RecipientTypeDetails TeamMailbox | Remove-Mailbox

WARNING: Site mailbox "O365-ExchangeConnections2015" couldn't be unlinked from the SharePoint site " Connections 2015" because of the following error: "SharePoint site "" couldn't be
contacted because of the following the error: "WebException - Status:ProtocolError; Message:The remote server returned an error: (404) Not Found.;HttpStatusCode:NotFound;HttpStatusDescription:Not

I’m Leaving the Problem to Microsoft

I lost interest at this point. The site mailboxes don’t bother me and I’m sure Microsoft will clean up the mess when they retire site mailboxes next April. After ignoring the trio of site mailboxes for so long, I can wait a little longer. In the meantime, to avoid seeing any further trace of the site mailboxes, I hid them by running:

Get-ExoMailbox -RecipientTypeDetails TeamMailbox | Set-Mailbox -HiddenFromAddressListsEnabled $True

Others might not be in the same state and need to transition off site mailboxes to Microsoft 365 Groups (the logical choice). If so, follow Microsoft’s advice to export the content from your site mailboxes and let this unsuccessful foray into on-premises server integration fade into the background.

We dropped site mailboxes from the Office 365 for IT Pros eBook in 2016. But we have tons of other great information in the only book which tracks changes across Office 365 and has done so since 2015.

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