The practical answer
In brief: an unlicensed shared mailbox can hold up to 50 GB. The people accessing it need licensed Exchange Online mailboxes. A shared mailbox needs additional licensing for a 100 GB primary mailbox, an expanded online archive, Litigation Hold, and certain advanced security or compliance features.
The three questions to answer before removing the licence
Check both. A 32 GB primary mailbox with an 80 GB online archive is not a “mailbox under 50 GB” for planning purposes.
What conversion does—and does not—do
Converting a user mailbox to shared preserves the mailbox and lets authorised internal users access it through their own licensed accounts. It does not turn the mailbox into a generic login.
A safer leaver workflow
1. Record primary mailbox size, archive size, Recoverable Items status, retention policies, and holds.
The three questions to answer before removing the licence
Check both. A 32 GB primary
- mailbox with an 80 GB online
- archive is not a “mailbox under
- 50 GB” for planning purposes.
- 50 GB” for planning purposes.
How large are the primary mailbox and archive?
Check both. A 32 GB primary mailbox with an 80 GB online archive is not a “mailbox under 50 GB” for planning purposes. The archive has its own entitlement requirements. Microsoft permits an unlicensed shared mailbox up to 50 GB. Assigning Exchange Online Plan 2 increases the primary mailbox limit to 100 GB. For archive capability and auto-expanding archiving, Microsoft documents Exchange Online Plan 2 or Exchange Online Plan 1 with the Exchange Online Archiving add-on. Must deleted content remain discoverable? Access for a manager and preservation for a legal matter are different requirements. If Litigation Hold is required, the shared mailbox needs Exchange Online Plan 2 or Exchange Online Plan 1 plus Exchange Online Archiving. Advanced Purview or Defender capabilities can require their own licences as well. Is this still an operational mailbox? A former employee mailbox that keeps receiving business email may be a shared operational mailbox for a transition period. A mailbox kept only so records remain discoverable may be better handled through a retention or inactive-mailbox design. Make that choice before deleting the user or removing entitlements.
What conversion does—and does not—do
Converting a user mailbox to shared preserves the mailbox and lets authorised
- internal users access it through their own licensed accounts. It does not
- turn the mailbox into a generic login. Microsoft says the associated shared-mailbox
- account is not intended for direct sign-in and should remain blocked.
- Conversion also does not grant unlimited storage, archive, hold, or security rights. The admin centre may allow a setting to
Practical checks
remain visible, and previously provisioned data may appear accessible for a time. That technical state is not proof of entitlement.
A safer leaver workflow
1. Record primary mailbox size, archive size, Recoverable Items status, retention policies, and holds. 2. Identify who needs ongoing access and for how long.
- 3. Decide whether the mailbox is operational, transitional, or evidence that must be preserved.
- 4. Convert to shared while the user mailbox is still licensed. 5. Confirm conversion and delegate access through licensed user accounts.
- 6. Block direct sign-in and remove unnecessary authentication methods and sessions.
- 7. Keep or assign the Exchange and compliance licences required by the chosen state. 8. Test access, inbound mail, auto-reply, forwarding, archive visibility, and discovery before closing the ticket.
Practical checks
9. Add a review or deletion date. “Forever” should be an explicit records decision, not an absent owner.
Common scenarios
A 20 GB mailbox is converted to shared, sign-in
- is blocked, two licensed colleagues
- receive access, and no
- hold or advanced feature is required. The shared mailbox
- normally does not need its own Exchange licence.
Small mailbox, no archive, short handover
A 20 GB mailbox is converted to shared, sign-in is blocked, two licensed colleagues receive access, and no hold or advanced feature is required. The shared mailbox normally does not need its own Exchange licence. Primary mailbox over 50 GB Do not remove the licence and hope the overage remains harmless. Either reduce the primary mailbox below the unlicensed limit through an approved records process or assign Exchange Online Plan 2 to support up to 100 GB. Archive must remain available Keep a qualifying archive entitlement. Microsoft identifies Exchange Online Plan 2 or Exchange Online Plan 1 with Exchange Online Archiving for the expanded archive and auto-expanding archive scenario. Litigation or regulated preservation License the mailbox for the hold and any additional Purview functionality used. Then verify the hold rather than assuming that conversion inherited the intended legal state. Long-term record, no operational use Assess whether an inactive mailbox is the more defensible architecture. That path has prerequisites: the correct retention or hold must exist before the user account is deleted. It is not a label that can be added after accidental deletion.
The misleading test: “But I can still see it”
Licensing audits fail when organisations equate successful configuration with licensed use. Exchange can retain previously provisioned properties, enforcement
- can be delayed, and an admin surface may expose a command that does not grant commercial rights.
- Use Microsoft Product Terms and current service descriptions as the entitlement test. Use the
- tenant state as evidence of configuration. Both matter, but they answer different questions.
- For organisations with many leavers, Axeti can help inventory shared mailboxes by size, archive, hold, feature use,
Practical checks
delegates, and last activity so that licences are retained only where the data requirement justifies them.
Practical checks
Sources and scope: Microsoft Learn, About shared mailboxes in Microsoft 365: 50 GB baseline, delegate licensing, sign-in model, archive and hold scenarios. Microsoft Learn, Exchange Online limits: mailbox and archive limits, user-access requirement, and archive-use restriction. Microsoft Learn, Place a mailbox on Litigation Hold: Plan 2 or Plan 1 plus Exchange Online Archiving requirement. Specific Purview and Defender features have benefit-based licensing rules beyond the mailbox storage licence. Validate the exact feature, user population, cloud, and agreement before publication or implementation.
