Someone hands in their notice, and their last day is Friday. Their address is printed on invoices, saved in a supplier portal and sitting at the top of three years of customer threads. So on Thursday afternoon somebody in a small company asks the question this page exists for: do we keep paying for their license so the mail keeps arriving, or do we turn the mailbox into a shared mailbox and stop paying?
Usually you convert it. But Microsoft's conversion guide has one sentence that most guides on this topic leave out: if you don't reset the password, "the original username and password will continue to work on the shared mailbox after the conversion is finished." Converting the mailbox doesn't lock anyone out. The person who left can still sign in.
This page covers the difference between a shared mailbox and a user mailbox in Microsoft 365: which one needs a license, where a shared mailbox runs out, and how to convert in both directions without losing mail or leaving that door open. Everything here was checked against Microsoft's current documentation in October 2026. There's a short Google Workspace section at the end. Two neighboring questions live on their own pages: whether you want a shared mailbox or a distribution list, and how members add the shared mailbox in Outlook.
The short version
- A user mailbox belongs to one person. That person signs in with their own credentials, and the mailbox always needs a license.
- A shared mailbox belongs to nobody. Members open it through their own mailboxes. It needs no license of its own up to 50 GB, as long as you don't put it on an archive, a litigation hold or certain compliance features. Every member still needs their own licensed mailbox.
- For a leaver, converting their mailbox to a shared mailbox keeps every email and calendar item. Leave the license on until the conversion is done, then reset the password, block sign-in and remove the license. Don't delete the account: the shared mailbox depends on it.
- Converting back to a user mailbox takes a few minutes. Assign a license and reset the password in the same sitting.
- Neither type knows who's handling a message. If several people answer from one address, that's a gap in both, and the licensing question doesn't solve it.
Shared mailbox vs user mailbox at a glance
| User mailbox | Shared mailbox | |
|---|---|---|
| Who signs in | One person, with their own username and password | Nobody directly. Members open it from their own mailbox, and sign-in on its account should stay blocked |
| License on the mailbox itself | Always | Not needed up to 50 GB, with no archive, litigation hold or advanced compliance features |
| Storage | Depends on the plan: 100 GB on Microsoft 365 Business Basic, Standard and Premium, 50 GB on Office 365 E1 and Exchange Online Plan 1 | 50 GB without a license, 100 GB with an Exchange Online Plan 2 license |
| Archive and litigation hold | Depends on the plan | Needs Exchange Online Plan 2, or Plan 1 plus the Exchange Online Archiving add-on |
| How many people use it | One owner, plus any delegates they allow in | Up to 25 members, each with their own licensed mailbox |
| People outside your company | Not applicable | Not possible, members must be inside your organization |
| Can you stop members deleting mail? | Only the owner and the delegates they let in have access | No, every member with Full Access can delete |
| Sending encrypted email | Depends on the plan | No, because the mailbox has no security context of its own |
| Best for | One person's mail | An address a team answers: support@, billing@, a leaver's old address |
The storage numbers come from Microsoft's Exchange Online limits page. The rest come from Microsoft's page about shared mailboxes. The sections below explain each row.
Two objects, one difference: whether a person signs in
A shared mailbox is a Microsoft 365 mailbox that no person signs in to: members open it from their own licensed mailbox, and it needs no license of its own up to 50 GB. A user mailbox is the opposite case.
A user mailbox is the normal kind. It's attached to one identity: one person, one username, one password, one license. Other people can get into it, but only because the owner or an admin gave them delegate access. That's a different setup from a shared mailbox, and delegate access to a person's own mailbox has its own page.
A shared mailbox looks like the opposite, but under the hood it isn't. A shared mailbox has a user account behind it too. In Microsoft's words: "Every shared mailbox has a corresponding user account with a system-generated password that isn't known or intended for use." New shared mailboxes are created with sign-in blocked, and Microsoft's instruction is to "always block sign-in for the shared mailbox account and keep it blocked." Members never use that account. They open the shared mailbox from inside their own licensed mailbox, using the permissions an admin gave them: Full Access to read it, plus Send As or Send on Behalf to reply from it.
The license is attached to whoever signs in. A user mailbox has a person signing in, so it needs a license. A shared mailbox has nobody signing in, so it doesn't, until it needs something only a license unlocks. The next section covers when that happens.
When a shared mailbox needs a license
No, not up to 50 GB. Microsoft's page says it directly: "A shared mailbox can store up to 50 GB of data without assigning a license to it." Four things change that.
- It grows past 50 GB. "To increase the size limit to 100 GB, the shared mailbox must be assigned an Exchange Online Plan 2 license."
- It needs an archive. A larger archive needs an Exchange Online Plan 2 license, or an Exchange Online Plan 1 license plus the Exchange Online Archiving add-on. Either one also lets you turn on auto-expanding archiving.
- It goes on litigation hold. It needs the same choice: Plan 2, or Plan 1 plus Exchange Online Archiving.
- You use advanced compliance features on it. Microsoft names Defender for Office 365, Microsoft Purview eDiscovery (Premium) and retention policies. Its two pages word this differently. The shared-mailbox page says "licenses might be required", while a footnote on the Exchange Online limits page says "the shared mailbox must be licensed for such feature(s)". Go with the stricter wording. If your tenant applies retention policies to mailboxes, check the shared ones as well.
One more inconsistency. Microsoft's page on creating a shared mailbox lists only Plan 2 for the first three cases. The shared-mailbox page also accepts Plan 1 with the Archiving add-on for archives and holds. If you already own Plan 1 licenses, that cheaper option is worth knowing about.
The shared mailbox costs nothing, but its members do. "To access a shared mailbox, a user must have a licensed Exchange Online mailbox, but the shared mailbox doesn't require a separate license." Check that the license includes email. Microsoft spells it out: "The Microsoft 365 Apps for business subscription doesn't include email."
If your tenant is older, check this too: "All unlicensed shared mailboxes created before July 2018 have a size of 100 GB." Those mailboxes got the larger limit without a license.
What happens when it fills up
Does a shared mailbox still receive email? Yes, until it's full. Microsoft's warning covers both stages: "When a shared mailbox reaches the mailbox storage limit, you can receive email for some time, but you can't send new email. After some time, the shared mailbox stops receiving email and senders to the mailbox receive a non-delivery receipt." Your team loses the ability to reply first, and the bounces come later. Customers usually notice before you do.
Where to check, and the billing trap
In the Microsoft 365 admin center, an unlicensed shared mailbox appears under Users > Active users with the status Unlicensed. That's also the only place you can remove a license from one. Microsoft says: "You can't remove the license from the Shared mailbox page because licenses are user settings." Select the mailbox, open the Licenses and Apps tab, expand Licenses, clear the box and select Save changes.
Then one more step, which Microsoft's note spells out: "You're still paying for the license. To stop paying for it, remove the license from your subscription." Unassigning the license frees it up, but you keep paying for it until you reduce the number of licenses in your subscription. If converting the leaver's mailbox was meant to save money, that's the second step.
What a shared mailbox can't do
These limits are all documented on Microsoft's shared mailbox page. Most teams run into one of them sooner or later.
- 25 members. "A shared mailbox supports a maximum of 25 users. If too many users access the mailbox at the same time, they might experience connection failures or duplicated messages." Microsoft's answer for more people is a Microsoft 365 group, and shared mailbox vs Microsoft 365 group vs distribution list explains that choice.
- Internal people only. "Only people inside your organization can use a shared mailbox." A freelancer or an agency on a personal Gmail address can't be a member.
- No encryption. "Email sent from a shared mailbox can't be encrypted because the mailbox doesn't have its own security context (username and password)." It gets worse: if members encrypt with their own keys, Microsoft warns that "other members might not be able to read those messages."
- No protection against deletion. "You can't prevent users from deleting messages in a shared mailbox." Every member with Full Access can delete anything. Microsoft's answer, if that's a problem, is again a Microsoft 365 group instead of a shared mailbox.
- Not a way to archive other people's mail. The limits page rules out one particular shortcut: an admin "can't create a shared mailbox and have users copy it (through the Cc or Bcc field, or through a transport rule) for the explicit purpose of archiving." If you need to keep mail for compliance, use retention policies.
There's one more limit that you won't find in Microsoft's documentation, and it comes from what a mailbox is for. A shared mailbox stores messages. It doesn't record who's dealing with them. There's no field that says "this one's mine", and nothing tracks how long a customer has been waiting for a reply. With two people working the mailbox, a quick word across the desk covers that. With five, a request gets answered twice or not at all, and nobody can tell which until the customer chases. That's the point where a shared mailbox stops being enough, and how a shared inbox works as a team practice picks up the story from there. A shared inbox tool like TriageFlow works on top of the same address and adds what the mailbox lacks: each incoming message can be assigned to one person, and the team can comment on the thread itself instead of forwarding it around.
How to convert a user mailbox to a shared mailbox
This is the leaver case, and the order of the steps matters. Microsoft's guide is convert a user mailbox to a shared mailbox. Here's the procedure with the warnings from that guide put in order.
Before you start
- Keep the license on. "The user mailbox needs a license assigned to it before you convert it to a shared mailbox. Otherwise, you won't see the option to convert the mailbox." If someone already removed it, add it back first.
- Check the size. You can only drop the license afterwards if the mailbox is under 50 GB. If it's bigger, Microsoft suggests deleting large messages, such as ones with attachments, until it fits.
The conversion
- In the Microsoft 365 admin center, go to Users > Active users.
- Select the user. On the Mail tab, select Convert to shared mailbox, then Convert.
If you run hybrid Exchange, read the hybrid section before you start: the conversion has to happen on-premises too, or the mailbox can be disconnected later.
Straight afterwards, in the same sitting
- Reset the password and block sign-in. This is the sentence from the top of the page: "if you don't reset the password, the original username and password will continue to work on the shared mailbox after the conversion is finished." For a leaver, Microsoft's guide adds that "you should take additional steps to make sure that they cannot log in anymore." Do both.
- Remove the license on the Licenses and Apps tab under Active users, as described above, and then reduce the licenses in your subscription if you want to stop paying.
- Don't delete the account. "Don't delete the old user's account, because the account is required to anchor the shared mailbox."
- Add the members who should handle the mail. They'll then add the shared mailbox in Outlook on their side.
Microsoft confirms what you keep: "all of the existing email and calendar information is retained", and "Inbox rules are preserved after the user mailbox is converted to a shared mailbox." That second point can catch you out. If the leaver had a rule that forwarded mail to a personal address, it's still running. Check their rules once the members have access.
If you already deleted the account
You can still recover, but not indefinitely. Microsoft's inactive-mailbox page says the mailbox data "is retained for 30 days after the account is removed", and undeleting the account in that window brings it back. Restore the user's account, make sure a license is assigned and reset the password. Microsoft says "it can take up to 24 hours for their mailbox to be re-created." Once it's back, convert it, remove the license and add members as above. Don't delete the account a second time.
Shared mailbox, forwarding, or inactive mailbox?
A shared mailbox is only one of three ways to handle a leaver's mail. Microsoft's offboarding guide names converting and forwarding side by side, and the third route, an inactive mailbox, is on Microsoft's Purview page linked below. Pick based on what you actually need afterwards.
| What you need | The right route |
|---|---|
| Someone keeps answering mail sent to that address, and can search the old threads | Convert to a shared mailbox |
| Only new mail matters, and it should reach a colleague | Forward the former employee's email to that colleague |
| Nobody answers it, but the mail has to be kept for legal or compliance reasons | Put a hold on the mailbox, then delete the account: it becomes an inactive mailbox |
The third option depends on the order of steps too. Microsoft's page on inactive mailboxes says that without a hold, "the employee's mailbox data is retained for 30 days after the account is removed", after which "the data is permanently removed". With a hold applied before you delete the account, "the mailbox will be converted into an inactive mailbox." Put the hold on first, then delete.
How to convert a shared mailbox back to a user mailbox
You'd go this way when a new hire takes over an address that someone had turned into a shared mailbox, or when a mailbox that started out shared now needs one person who signs in. It's done in the Exchange admin center, not the Microsoft 365 admin center.
- Go to the Exchange admin center and select Recipients > Mailboxes.
- Select the shared mailbox and select Convert to regular mailbox. Microsoft's two guides put this button in different places. The admin-center guide says it's on the Others tab, while the Exchange Online conversion page says it's in the More actions pane. Neither page explains the difference, so look for it in both places.
- Select Confirm.
- Back in the Microsoft 365 admin center, go to Users, open the account linked to the old shared mailbox, assign a license and reset the password.
Microsoft says "it will take a few minutes for the mailbox to get set up". When the new owner signs in, "they'll see the email and calendar items that used to be in the shared mailbox." Nothing gets moved or re-imported, because it's the same mailbox with a different type.
If you'd rather use Exchange Online PowerShell, it's one line. Microsoft's example is Set-Mailbox -Identity MarketingDept1 -Type Regular, with your mailbox's name in place of MarketingDept1. Running the same command with -Type Shared converts in the other direction.
License the account straight after you convert it, in the same sitting. A regular mailbox without a license is a risk: Microsoft's hybrid troubleshooting article, covered next, says Exchange Online "might disconnect a regular mailbox if it has no license assigned and no hold applied."
Hybrid setups: change the type on-premises too, in both directions
Everything above assumes a cloud-only tenant. If your organization runs hybrid Exchange (on-premises servers synced with Exchange Online), the Exchange Online conversion page says "you need to manage your mailboxes using on-premises Exchange management tools", and that applies to the leaver case as much as to converting back.
For a mailbox that was created in Exchange Online but never migrated there, you change the type on-premises with Set-RemoteMailbox <MailboxIdentity> -Type <Regular | Shared | Room | Equipment>. The same page adds the sentence that matters for most hybrid leavers: "If the mailbox was previously migrated to Exchange Online, you also need to update the mailbox object in Exchange Online." So a migrated mailbox needs the change in both places.
Skip the on-premises half and Microsoft's troubleshooting article shared mailbox in Exchange Online is unexpectedly disconnected describes what happens, and its scenario is exactly the leaver one: a migrated user mailbox converted to shared in Exchange Online, unlicensed, with no hold. Because sync runs from on-premises to the cloud, "the mismatch in the mailbox type can cause Exchange Online to later revert the shared mailbox to a regular mailbox. Exchange Online might disconnect a regular mailbox if it has no license assigned and no hold applied." The deadline is strict: "Permanent loss of the mailbox contents can occur if these steps aren't completed within 30 days from the time that the mailbox was disconnected."
Microsoft's fix has four steps: temporarily assign a license to reconnect the mailbox, run Set-RemoteMailbox -Identity <mailbox ID> -Type Shared in the on-premises Exchange Management Shell, run Set-Mailbox -Identity <mailbox ID> -Type Shared in Exchange Online PowerShell, then remove the temporary license. Doing the on-premises change at conversion time saves you all four.
Google Workspace: the shared inbox and converting an account
Google Workspace didn't have this choice for a long time, and many pages still say it has no shared mailbox. That's out of date. Google now offers a shared inbox in the Admin console: a mailbox an admin creates and gives people access to as delegates, on Business Starter, Business Standard and Business Plus. Google is still rolling it out in phases, and says that if you don't see it yet, you should use delegated Gmail accounts instead.
The closest match to this page's topic is on the same help page: you can convert an existing email account in your organization, including a delegated one, into a shared inbox. You do it from the Admin console's Users section with Set up a shared email address. Email sent from it shows the shared address as From and the delegate as Sent by, unless an admin hides the Sent by information. Google's page doesn't say whether the shared inbox takes up a paid license, so check your own Admin console and billing before you count on the saving.
If your edition doesn't have it yet, the four ways to share an inbox in Gmail compares the alternatives. Google Groups and Collaborative Inbox are the older routes, and that page covers them too.
Frequently asked questions
Does a shared mailbox still receive emails?
Yes, and converting a leaver's mailbox doesn't change that: mail to their address keeps arriving in the new shared mailbox. The exception is a full mailbox. At the storage limit, the mailbox keeps receiving for a while but can't send. After that, it stops receiving and senders get a non-delivery receipt.
Can you sign in to a shared mailbox directly?
You can, but you shouldn't. Every shared mailbox has a user account with a password nobody's supposed to know, and Microsoft's instruction is to block sign-in on it and keep it blocked. The usual way people end up signing in by accident is a converted leaver mailbox where nobody reset the password. In that case the old credentials still work.
What happens to the license when you convert a mailbox?
Nothing happens to it automatically. Converting a user mailbox to shared requires the license to stay on, and you remove it yourself afterwards (under 50 GB, with no archive or hold). Even then you're billed for it until you reduce your subscription. Going the other way, Microsoft's steps have you assign a license to the account straight after the conversion.
Will converting a shared mailbox to a user mailbox delete the email?
No. It's the same mailbox with a different type, so the person who signs in afterwards sees the email and calendar items that were in the shared mailbox. The one real risk is in hybrid setups. If the type isn't changed on-premises too, Exchange Online can revert and disconnect the mailbox, and Microsoft gives you 30 days to fix it before the contents are lost.
What are the disadvantages of a shared mailbox?
Microsoft lists a maximum of 25 members, no access for people outside the organization, no encrypted email and no way to stop members deleting messages. In practice the bigger problem is that nobody owns a message. The mailbox shows everyone the same messages, but it can't show who's answering which one.
Can people outside the company use a shared mailbox?
No. Only people inside your organization can be members, so a contractor on a personal address can't be added. If outside people need to be part of the conversation, you're looking at a different object, usually a group.
Bottom line
Use a user mailbox when one person signs in, and a shared mailbox when a team answers an address and nobody signs in. When someone leaves, convert, then reset the password, keep the account and reduce the subscription. And if the real problem behind your question is that several people answer from one address and nobody knows who's got which message, changing the mailbox type won't solve it.