Shared mailbox in Microsoft 365: when to use one instead of a licence
A shared mailbox is a mailbox that several people open from inside their own Outlook, with no separate password to remember, and it costs nothing extra to run up to 50 GB. That last part is the reason it matters commercially: addresses like info, accounts, sales and support do not need a paid licence each, and neither does the address of someone who has left. Used properly it is one of the few genuine savings in Microsoft 365 that does not cost you anything in capability.
Last updated 14 August 2026. We review this guide every six months, and after any significant change to the Microsoft 365 admin centre.
Creating or changing a shared mailbox needs administrator access. Specifically an Exchange admin role. If you open the shared mailboxes page and are told you do not have permission to access it or perform the action, you are not an admin, and you will need whoever holds that role. Opening and using a shared mailbox that already exists needs nothing special.
What it actually is
A shared mailbox is a mailbox with no usable sign-in of its own. Behind the scenes it has an account attached, but that account has a system generated password nobody knows, and sign-in to it is blocked by default and should stay blocked. Instead of logging into it, people sign in as themselves and the shared mailbox appears alongside their own.
Everyone sees the same mail, so there is one copy of the conversation rather than four people forwarding each other the same enquiry. It comes with a shared calendar as well, which is often the quiet reason a team ends up liking it more than they expected.
The money question, answered honestly
This is the part worth being precise about, because it is often oversold.
What you save: the mailbox itself is free up to 50 GB. Every generic address you run this way, and every departed employee's address you convert rather than keep paying for, is a licence you are not buying. For a small business with an info address, an accounts address, a sales address and two people who left in the last few years, that is five licences of ongoing cost removed.
What you do not save: every person who opens the shared mailbox still needs their own Exchange Online licence. A shared mailbox is not a way to give five staff email on one licence. The saving is on the mailbox, not on the seats, and anyone who tells you otherwise is going to leave you non compliant.
The single clearest win is the staff departure case. When someone leaves, their mailbox can be converted to a shared mailbox: the address keeps working, the history stays intact, the team keeps receiving anything sent to them, and the licence stops being paid for. The common alternative, quietly leaving a licensed account running for years so nothing gets missed, is a cost plenty of businesses carry without ever putting a number on it.
When a shared mailbox is the wrong answer
It is not the tool for every job, and knowing the limits saves you a rebuild later:
- You need more than 50 GB. Beyond that the mailbox needs an Exchange Online Plan 2 licence, which takes it to 100 GB. At that point compare it honestly against other options.
- You need someone outside the business to have access. Shared mailboxes are internal only. You cannot grant access to an outside address such as a personal account.
- Someone needs to sign into it directly. That is not what it is for, and blocking sign-in on it is a security control worth keeping rather than working around.
- You need litigation hold or a large archive. Both need Plan 2, or Plan 1 with an archiving add-on.
- You want to turn it into a Microsoft 365 group later. A shared mailbox cannot be migrated to a group, so if a group is what you actually want, start there.
The three permissions, and why the difference matters
Nearly every shared mailbox problem we get called about traces back to this, so it is worth understanding once:
| Permission | What it allows | How sent mail appears |
|---|---|---|
| Full Access | Open the mailbox, read, file, delete, manage the calendar. Does not allow sending. | Cannot send at all |
| Send As | Send as the mailbox itself. | From the Accounts Department |
| Send on Behalf | Send on the mailbox's behalf, with your own name shown. | John on behalf of Reception |
Full Access on its own does not let anyone send. That is the answer to the most common complaint about shared mailboxes, which is that someone can read the mail perfectly well but their replies will not go. They need Send As or Send on Behalf added as well. Most business shared mailboxes want Full Access plus Send As, so replies come from the department rather than from an individual.
Choose deliberately between the two sending permissions. Send As is right for a customer facing address, where the customer should be dealing with your business rather than a specific staff member. Send on Behalf is better where accountability matters and you want it visible who actually replied.
Setting one up
In the Microsoft 365 admin centre, go to Teams & Groups and then Shared mailboxes. If you cannot see Teams & Groups, choose Show all in the left navigation. Select Add a shared mailbox, give it a name, which sets the address, and adjust the address if the generated one is not what you want. Save it.
Give it a minute. It is normal for a new shared mailbox to take a short while before members can be added, and people often assume something has failed when nothing has. Then use Add members to this mailbox to choose who should see the incoming mail and the replies.
For finer control over the permission types, the Exchange admin centre is the better place: Recipients, then Mailboxes, select the shared mailbox, edit it, and under Mailbox permissions choose Manage mailbox delegation. That is where Full Access and Send As are granted or removed individually.
Why it does not appear in Outlook
Once permissions are granted, the mailbox normally appears in a member's Outlook by itself. That behaviour is called automapping and it is on by default. When it does not happen, it is almost always one of these:
- Outlook has not been restarted. Automapping adds the mailbox when Outlook next opens, not instantly. Close it fully and reopen it.
- Access was granted through a security group. This is the one that catches experienced people out. Automapping is set on the individual user's mailbox, so granting access via a group means automapping never happens. If you want the mailbox to appear automatically, assign the permission to people directly. If you prefer to manage access by group, that is a defensible choice, but expect to add the mailbox manually in Outlook.
- The permission has not finished applying. Changes are not always instant. If everything looks right, give it a little time before rebuilding anything.
There is one more that is easy to miss: if the mailbox has been hidden from the address list, Send As and Send on Behalf stop working in the Outlook desktop program, because those permissions need the mailbox to be visible in the global address list. If someone hid it to reduce clutter and sending broke shortly afterwards, that is your cause.
The failure that looks like a broken mailbox
If a shared mailbox has gone quiet, check its size before you check anything else. A full shared mailbox does not stop cleanly: it carries on receiving for a while but can no longer send, and then it stops receiving as well and the people writing to it start getting bounce messages. From the inside it just looks like the address went dead.
Fifty gigabytes sounds like a lot until you remember that an accounts address has been receiving supplier PDFs since 2015. If that is where you are, the choices are to archive the history properly, or to licence the mailbox with Plan 2 and take it to 100 GB. If people are getting bounces from it, the bounce message itself will name the reason, which saves guessing.
If the shared mailbox is receiving but individuals cannot see new mail, that is a different problem and it is worth working through why Outlook is not receiving emails instead, because the cause is more likely to be a rule or a filter than the mailbox itself.
FAQ
Does a shared mailbox need a licence?
Not for normal use. A shared mailbox holds up to 50 GB without any licence assigned to it, which is why it is the standard way to run addresses like info or accounts without paying for them. It needs an Exchange Online Plan 2 licence to go beyond 50 GB, to grow its archive, or to be placed on litigation hold. The part people miss is that every person who opens the shared mailbox must have their own Exchange Online licence, so the saving is on the mailbox, not on the staff.
Why can I open the shared mailbox but not send from it?
Because opening and sending are two separate permissions. Full Access lets you open the mailbox, read it, and manage what is in it, but it does not let you send. To send you also need Send As, which makes the mail look like it came from the mailbox, or Send on Behalf, which shows your name alongside it. This is the single most common shared mailbox support call, and the fix is to add the second permission rather than anything on the computer.
Why is the shared mailbox not showing in Outlook?
Usually one of three things. Outlook has not been restarted since the permission was granted, and automapping only adds the mailbox on restart. Or permissions were granted through a security group, which breaks automapping entirely, because automapping is set on the individual user's mailbox. Or the permission simply has not finished applying yet, which can take a little time after the mailbox is created. If access was given via a group, either assign it to people directly or add the mailbox manually in Outlook.
Can someone outside my business use a shared mailbox?
No. Shared mailboxes are for people inside your organisation only, so you cannot grant access to an outside address such as a personal Gmail account. If you need to include a contractor or an external partner, the usual answers are to give them a proper account in your organisation, or to use a group for Outlook instead, depending on what they actually need to do.
What happens when a shared mailbox gets full?
It fails gradually, which is why it catches people out. When the mailbox reaches its storage limit it keeps receiving mail for a period but can no longer send. After that it stops receiving too, and people writing to it start getting bounce messages. An address that has been collecting mail for years can reach 50 GB without anyone noticing, so it is worth checking the size of any shared mailbox that has suddenly gone quiet.
Should we use a shared mailbox for someone who has left?
Usually yes, and it is the clearest saving available. When a staff member leaves you can convert their mailbox to a shared mailbox, which keeps the address alive so their mail still reaches the team, preserves the history, and removes the need to keep paying for a licence. The alternative, leaving a licensed account running indefinitely so nothing is missed, is a cost a lot of small businesses carry for years without realising it.
The bottom line
Use a shared mailbox for any address that belongs to a role rather than a person, and for anyone who has left. It is free up to 50 GB, everyone sees the same conversation, and there is no extra password in circulation. Remember that the people using it still need their own licences, that Full Access alone will not let anyone send, and that granting access through a security group will stop it appearing in Outlook by itself. Watch the 50 GB limit on old addresses, because that is how a working mailbox goes silent without any obvious cause.
Not sure whether you are paying for licences you do not need? We will look at how your mailboxes are set up and tell you what can be converted. Ask us to take a look and we will give you a straight answer.