Comparing Shared and Inactive Mailboxes

Options for Dealing with Leaver Mailboxes

When someone leaves an organization, a discussion often takes place about what to do with their mailbox and other data. For Exchange Online, the choice is straightforward:

  • Delete mailboxes.
  • Keep the mailboxes and let someone else take over the Azure AD accounts (and mailboxes).
  • Change mailboxes to become shared mailboxes.
  • Preserve them as inactive mailboxes.

Usually, the choice comes down to either a shared or inactive mailbox. Of course, the mailboxes belonging to ex-employees store other personal information in places like OneDrive for Business and Teams chat. Other information, like the documents kept in SharePoint Online sites, is by definition shared and remains accessible to other users. This discussion focuses on what to do about “leaver” mailboxes.

Shared Mailboxes

Shared mailboxes have existed in Exchange for a long time and are well understood. The advantages of transforming a user mailbox to be a shared mailbox are:

  • The mailbox remains online and is accessible using any Outlook client. It appears in Exchange address lists like the GAL and can continue to receive inbound emails.
  • Users can receive permission to access and recover mailbox contents. If necessary, administrators can grant users Send As and Send on Behalf Of permissions to allow them to send emails from the shared mailbox.
  • When a user mailbox becomes shared, it no longer needs an Exchange Online license unless it is larger than 50 GB or has an archive.
  • If necessary, administrators can easily change the mailbox back to become a regular user mailbox. At this point, it must have an Exchange Online license.

Changing a mailbox to be shared is a good approach when it’s necessary for other users to take over responsibility for the work of a departed employee. For example, the manager of a sales representative who leaves the organization needs to follow up on customer engagements and commitments. Privacy can be a big concern when someone gains access to another person’s mailbox because there’s probably some personal material among business-related emails. For this reason, organizations often limit access to a mailbox for a set period after which the mailbox is deleted.

Inactive Mailboxes

In an on-premises organization, it doesn’t matter if leaver mailboxes remain online. Licenses are not required because no one uses the mailboxes. If storage is available, leaver mailboxes can stay in place for as long as the organization wishes.

The situation is different within Office 365 as Exchange Online removes unlicensed mailboxes soon after the deletion of their owner’s Azure AD accounts. To make it possible for organizations to retain leaver mailboxes for compliance purposes, Microsoft introduced inactive mailboxes several years ago. If a hold applies to a mailbox or retention labels with holds exist on items in a mailbox, Exchange Online won’t delete the mailbox following the removal of its owner’s account. Instead, Exchange Online puts the mailbox into a hidden and inactive state. The content of the mailbox remains indexed and discoverable and can be found by eDiscovery searches.

The important things to remember about inactive mailboxes are:

  • Inactive mailboxes remain online until the last hold (policy or retention label) lapses or an administrator removes a litigation hold on the mailbox. At this point, Exchange Online will retain the mailbox in a soft-deleted state for a further 183 days and then permanently removes the mailbox. Inactive mailboxes don’t need any type of license.
  • Inactive mailboxes are invisible to normal client interfaces, like OWA and Outlook desktop. They do not appear in Exchange address lists and cannot receive new emails.
  • The complete content of a mailbox remains available when it becomes inactive, including its archive and the compliance records captured by the Microsoft 365 substrate for Teams, Yammer, and Planner.
  • To access mailbox content, administrators must either recover or restore an inactive mailbox. Recovering an inactive mailbox makes it active and usable again. Restoring means that material from the inactive mailbox (or its archive) is merged into another mailbox.

Essentially, inactive mailboxes are a compliance tool. They facilitate long-term storage of mailbox content to ensure that the material in the mailboxes remains accessible if necessary. Inactive mailboxes are a good way to keep mailboxes of senior employees and other staff subject to regulatory oversight for extended periods. Figure 1 shows a tenant with shared mailboxes going back to February 2015 as viewed through the Microsoft 365 Purview portal.

Inactive mailboxes in the Microsoft Purview compliance portal
Figure 1: Inactive mailboxes in the Microsoft Purview compliance portal

If you have the licenses needed to use adaptive scopes with Microsoft 365 retention policies, you can create a user scope for inactive mailboxes. If the organization has the need to keep mailboxes for an extended period (say, five years), it’s a good idea to create a retention policy with a five-year retention period and an adaptive scope targeting inactive mailboxes. That way, even if the retention period for other holds and retention labels expire, you’ll know that Exchange Online will retain the inactive mailboxes for the required period.

The Choice is Clear

GUI access to inactive mailboxes is via the Microsoft Purview compliance portal. That gives you a good clue about the essential choice between inactive and shared mailboxes. If you want to keep information because it’s needed to satisfy some regulatory or legal requirements, use inactive mailboxes. But if the organization needs information in a mailbox for immediate business reasons, transforming a leaver mailbox into a shared mailbox is a better choice.


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