An Outlook shared mailbox lets several people in your business watch and send from one address, such as info@ or support@, and it needs no licence of its own up to 50 GB. Replies look like they came from the shared address, and it comes with a shared calendar too.
Here's the catch. That free mailbox stops at 50 GB and 25 people, and nobody outside your organisation can use it. Once you hit any of those walls, a Microsoft 365 group or a licence is the better answer. This is how it works as of October 2026, set out for a small business owner rather than an IT department.
Shared mailbox limits at a glance
| Setting | What Microsoft allows |
|---|---|
| Storage without a licence | 50 GB |
| Storage with Exchange Online Plan 2 | 100 GB |
| Maximum users | 25 |
| People outside your organisation | Not allowed |
| Sign-in to the mailbox account | Blocked by default |
| Encrypted mail sent from it | Not possible |
| Convert a user mailbox to shared | Yes |
The licence point trips people up. The shared mailbox doesn't need one, but everyone who opens it does need a licensed Exchange Online mailbox of their own. Microsoft 365 Apps for business doesn't include email, while Business Standard does. So if your team is on the Apps-only plan, they can't use a shared mailbox until that changes.
How to create a shared mailbox in the admin center
You'll need the Exchange admin role for this. Then:
- Sign in to the Microsoft 365 admin center.
- Go to Teams & groups, then Shared mailboxes, and select + Add a shared mailbox.
- Give it a name and save it.
- Select Add members. It can take a few minutes before members can be added, so don't panic if the option isn't ready straight away.
- Grant Full Access plus Send As or Send on Behalf, so people can reply as well as read.
If someone's left and you want to keep their inbox running, you don't have to start from scratch. You can convert a user mailbox to a shared mailbox. Once it's set up, my walkthrough on adding a shared mailbox to Outlook covers the desktop side for your team.
Full Access, Send As or Send on Behalf
These three permissions decide what your team can do, and getting them wrong is the most common reason someone says the inbox is broken.
- Full Access lets a person open the mailbox and act as its owner. On its own it doesn't let them send.
- Send As makes the email look like it came straight from the shared mailbox.
- Send on Behalf shows the sender's name too, as in John on behalf of the mailbox.
For a support@ or hello@ inbox, I'd give Full Access plus Send As, because customers then see one consistent address. Use Send on Behalf when you want the customer to know which person answered.
One quirk to know. Send As and Send on Behalf don't work in the Outlook desktop app if the mailbox is hidden from address lists. If someone can read but can't send from their desktop, check that setting before anything else.
What happens when it fills up
When a shared mailbox reaches its storage limit, it keeps receiving for a while but can't send. After some time it stops receiving too, and anyone writing in gets a non-delivery receipt. For a sales or support inbox, that's lost enquiries you'll never see.
To go to 100 GB you'll need an Exchange Online Plan 2 licence on the mailbox. Archive growth or litigation hold needs Plan 2, or Plan 1 plus the Exchange Online Archiving add-on.
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Security rules worth keeping
- Sign-in is blocked by default for the account behind a shared mailbox, and it should stay blocked. The password the system generates isn't meant to be used.
- Mail sent from a shared mailbox can't be encrypted, because it has no security context of its own. Don't use it for anything that has to go out encrypted.
- You can't stop people deleting messages in it. Anyone with access can remove an email for everyone.
When a Microsoft 365 group is the better choice
Microsoft points you to a Microsoft 365 group instead of a shared mailbox in three cases:
- You've got more than 25 people who need the inbox. Too many users at once can cause connection failures or duplicated messages.
- You need to stop people deleting messages.
- You need to give outsiders, such as a freelancer on Gmail, access.
Decide early, because you can't migrate a shared mailbox to a Microsoft 365 group later. If all you want is one address that forwards to a few people and nobody replies from it, a distribution list in Outlook may be all you need.
My verdict on the shared mailbox
I'd use a shared mailbox for any small team answering a general inbox, as long as everyone's inside the business and on a plan that includes email. Set the permissions right on day one, leave sign-in blocked, and keep an eye on the 50 GB. Skip it if freelancers or outside partners need to reply, or if your team's heading past 25 people, because a Microsoft 365 group handles both and you can't switch over later.
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Frequently asked questions
Does an Outlook shared mailbox need a licence?
The shared mailbox doesn't need one up to 50 GB, as of October 2026. Everyone who uses it needs a licensed Exchange Online mailbox of their own.
How many people can use a shared mailbox?
A maximum of 25 users. Too many people at once can cause connection failures or duplicated messages.
Can someone outside my company use a shared mailbox?
No. Only people inside your organisation can use it. Microsoft suggests a Microsoft 365 group if outsiders need access.
Why can't I send from a shared mailbox?
Full Access alone doesn't let you send. You also need Send As or Send on Behalf, and both fail in Outlook desktop if the mailbox is hidden from address lists.
