When an employee leaves your business or changes roles, their mailbox often contains years of valuable business information: customer conversations, supplier correspondence, project details, appointments and contacts.
Deleting the account immediately can create unnecessary risk. Keeping a fully licensed user mailbox indefinitely, however, may mean paying for a licence that nobody needs.
Converting the user mailbox into a Microsoft 365 shared mailbox can provide a practical middle ground. Your business can preserve the mailbox data, allow authorised team members to access it and potentially remove the user licence.
This guide explains the process in straightforward steps for SMEs. If you prefer expert assistance, Anantek Solutions provides IT support for small business and ongoing Managed IT services.
When Should You Convert a User Mailbox?
Converting a mailbox makes sense when the address or its contents still have business value, but the original employee no longer needs a personal Microsoft 365 account.
Typical examples include:
- A sales manager leaves, but the business needs access to their customer history.
- An employee changes departments and their old address must remain available.
- A director retires, but their mailbox contains important supplier or client correspondence.
- A business wants a team to manage messages sent to a former employee’s address.
- You want to preserve email data without paying for an unused user licence.
A shared mailbox allows several authorised people to access the same mailbox through their own Microsoft 365 accounts. This is more secure than sharing the former employee’s password and makes access easier to control.
When an Alternative May Be Better
Conversion is not always the right option.
Consider keeping the mailbox as a licensed user mailbox if one specific employee will take full ownership of the account and needs to sign in as that user.
A forwarding rule may be suitable when you only need new messages redirected elsewhere. However, forwarding does not provide the same convenient access to historical email, calendar entries or contacts.
For regulatory, legal or employment-related requirements, you may need a formal retention, eDiscovery or legal-hold strategy instead. A shared mailbox is not a replacement for a backup system, retention policy or compliance archive.
Before You Start: Permissions and Licence Requirements
You need an account with sufficient administrative permissions in Microsoft 365. In most SMEs, this will be a Global Administrator or an administrator with the relevant user and Exchange permissions.
There is also an important licensing requirement:
The user mailbox must have an Exchange Online licence assigned before you convert it.
If the licence has already been removed, the conversion option may not appear. In that situation, temporarily assign an appropriate Microsoft 365 or Exchange Online licence, wait for the mailbox to become active, and then complete the conversion.
Microsoft’s official mailbox conversion guidance confirms that the existing mailbox data is retained during the process.
Check the Mailbox Size
An unlicensed shared mailbox is generally limited to 50 GB. If the mailbox is close to or above that limit, review its size before removing the licence.
A licence may still be required where the mailbox needs:
- More than 50 GB of storage.
- An archive mailbox or auto-expanding archive.
- Litigation Hold or other advanced compliance features.
- Specific retention or legal-hold capabilities.
Do not delete the original user account after conversion. Microsoft 365 needs the account to remain in place as the identity anchor for the shared mailbox.
Step 1: Convert the Mailbox in the Microsoft 365 Admin Center
Once you have checked the licence, mailbox size and business requirements, you can complete the conversion.
- Sign in to the Microsoft 365 Admin Center.
- In the navigation menu, select Users.
- Select Active users.
- Find and select the user whose mailbox you want to convert.
- Open the Mail tab in the user details pane.
- Select Convert to shared mailbox.
- Select Convert to confirm.
The wording and layout may vary slightly depending on Microsoft 365 updates, but the process remains broadly the same.
The conversion may not appear immediately if the account does not have a suitable Exchange Online licence. If you have just assigned a licence, allow time for Microsoft 365 to provision the mailbox before trying again.
Step 2: Understand What Happens to the Mailbox Data
Converting a mailbox changes its type. It does not normally remove the existing mailbox content.
The following information is retained within the converted mailbox:
- Existing email messages.
- Folder structures.
- Calendar appointments and meetings.
- Contacts.
- Inbox rules.
- Mailbox history and business correspondence.
The difference is that the mailbox is no longer treated as a private mailbox belonging to one active user. Instead, it becomes a shared resource that authorised team members can access.
The original email address normally remains available. This is especially useful when customers, suppliers or partners still use the former employee’s address. You can also add aliases later if the business wants the mailbox to receive messages sent to another address.
Before making changes, it is still sensible to document the mailbox state and confirm that critical data is protected by your normal backup, retention and security processes.
Step 3: Assign the Right Permissions
A converted mailbox is not automatically useful to the wider team. You must give selected users access.
In the Microsoft 365 Admin Center:
- Go to Teams & groups.
- Select Shared mailboxes.
- Select the converted mailbox.
- Open the Members section.
- Select Edit or Add members.
- Choose the people who need access.
- Save your changes.
Depending on your Microsoft 365 interface, you may also manage permissions through the Exchange Admin Center.
The main permissions to understand are:
Full Access
Full Access allows a user to open the shared mailbox, read messages, organise folders and manage mailbox contents.
It does not automatically allow the user to send messages from the shared address.
Send As
Send As allows a team member to send an email that appears to come directly from the shared mailbox, such as accounts@yourbusiness.com.
This is usually the preferred option for customer-facing departmental mailboxes.
Send on Behalf
Send on Behalf shows the individual sender as sending on behalf of the mailbox. For example, the recipient may see “Alex Smith on behalf of Accounts”.
Choose the permission model that matches your business process. Avoid giving every employee Full Access by default. Access should be limited to people who need it for their role and reviewed regularly.
Step 4: Block the Former User’s Sign-In
If the employee has left the company, do not assume that conversion alone prevents access.
The original username and password may continue to work after conversion unless you take additional action. Before removing the licence, or as part of your offboarding process:
- Block sign-in for the former user account.
- Reset the password if appropriate.
- Revoke active sessions and tokens.
- Remove the user from unnecessary groups and applications.
- Remove their access from the shared mailbox if they should no longer use it.
- Confirm that company devices and mobile access have been recovered or secured.
Do not delete the account. The account must remain in Microsoft 365 as the anchor for the shared mailbox.
These steps should form part of a wider employee offboarding checklist covering Microsoft 365, devices, VPN access, file storage, Teams and third-party applications.
Step 5: Decide What to Do with the User’s Licence
After conversion, you can usually remove the user’s Microsoft 365 licence if:
- The shared mailbox is below the applicable unlicensed storage limit.
- Advanced archive or compliance features are not required.
- No other services depend on that licence.
- The user account remains in place.
- Sign-in has been blocked where appropriate.
To remove the licence:
- Open Users > Active users in the Microsoft 365 Admin Center.
- Select the former user.
- Open the Licences and apps section.
- Unassign the relevant Microsoft 365 or Exchange Online licence.
- Save the changes.
Check the account carefully before removing the licence. Some Microsoft 365 plans include applications or services beyond email. Removing the wrong licence could affect access to services the business still needs.
Step 6: Complete Post-Conversion Checks
Allow time for permissions and mailbox changes to synchronise. Then ask an authorised team member to test the mailbox.
Check that:
- The shared mailbox appears in Outlook.
- Team members can open the mailbox.
- Existing email can be found.
- Calendar entries are present.
- Contacts remain available.
- Users can send as the shared mailbox where required.
- Replies are saved in the shared mailbox’s Sent Items folder.
- New messages are arriving correctly.
- Mobile and Outlook on the web access work as expected.
- The former employee cannot sign in.
Sometimes the mailbox does not appear immediately in Outlook. Restarting Outlook or manually adding the shared mailbox may help. Permission changes can also take time to propagate across Microsoft 365.
Best Practices for Ongoing Shared Mailbox Management
A good shared mailbox setup is more than a one-time conversion. The mailbox should support a clear team workflow.
Add a Useful Auto-Reply
If customers may still contact the former employee, create an automatic reply explaining that the mailbox is now monitored by a team. Include a replacement contact where appropriate.
Review Aliases
Keep the former employee’s address if it is still needed, but consider adding a role-based alias such as sales@, projects@ or accounts@. This helps move communication away from an individual address over time.
Organise the Inbox
Use folders such as:
- New enquiries
- In progress
- Waiting for information
- Completed
Agree a simple process so the team knows who is handling each message and when an email can be archived.
Enable Strong Security
Require Multi-Factor Authentication on every individual user account accessing the shared mailbox. Never distribute a shared password or allow team members to sign in directly as the mailbox.
Review members and permissions at least quarterly, and immediately after role changes.
Protect the Wider Environment
Mailbox security depends on the wider Microsoft 365 environment. Regular reviews should include conditional access, admin accounts, backup, retention, endpoint security and user access.
The same principle applies to the wider technology estate. Whether Anantek is optimising a school’s network, delivering structured cabling and Wi-Fi installations for a fit-out company, or supporting infrastructure for high-end retail brands such as Audemars Piguet (AP) and A. Lange & Söhne (ALS), reliable technology should work quietly in the background.
That is the value of Invisible Infrastructure: systems that remain secure, dependable and ready for the people who rely on them.
Convert with Confidence
Converting a user mailbox to a shared mailbox is a practical way to preserve business data, support continuity and reduce unnecessary licensing costs. The key is to plan first, keep the original account, assign permissions carefully and complete the security checks after conversion.
If your business needs help with a Microsoft 365 shared mailbox, wider Managed IT services, network modernisation or a structured IT review, contact Anantek Solutions. Our team provides responsive support, with an average response time of one minute, helping SMEs build secure technology that lasts.
