Outlook Shared Mailbox: Running Support Out of One, and Where It Stops

An Outlook shared mailbox is a Microsoft 365 mailbox with no user of its own, which several people open alongside their own mail and answer from using the shared address. It costs nothing on top of the licences you already hold, it holds 50 GB, and Microsoft supports a maximum of 25 users on one. A support desk of three or four can run on it for years without any trouble at all. Plenty do.

Where it stops is documented, mind you, and Microsoft is not hiding any of it. The numbers sit in two pages on their own site and they are plain enough once somebody goes and reads them, which is a thing almost nobody does before the mailbox is already carrying the whole of the customer mail and four people’s habits along with it.

We sell a help desk, so a page like this one written by us wants reading with a bit of salt on it. Fair is fair though, a good part of what follows is a setup guide for staying exactly where you are, and if you are a desk of three we would rather you kept your money.

What an Outlook shared mailbox actually is

Say support@ is the address and four of you answer it. What Microsoft gives you is a mailbox with a disabled account sitting behind it, no password anybody knows, no sign-in at all, and permissions handed out to your own licensed accounts so the thing appears in Outlook underneath your own mail without anybody typing another credential into anything. That is the whole of it, mechanically speaking, and it is the same in Office 365 tenants that never went and changed their name over.

Three permissions do all the work. Full Access opens the mailbox and lets you read and file and delete in it. Send As puts the reply out as support@ with nothing to say a person was involved. Send on Behalf puts your own name on it beside the address, which some desks prefer and most never go and turn on. There is no licence to buy for any of it. The account underneath must have its sign-in blocked and kept blocked, mind you, which Microsoft says twice over on the one page and they are right to say it twice.

Now the part that decides the rest of it. A shared mailbox is a place where mail sits. It is not a process and it never once claimed to be one, truth be told. Nothing in there knows which of you is dealing with a message. Nothing is counting how long anybody has waited either, and nothing whatever will stop the two of you opening the same thing at half nine on a Monday and typing two different answers into it.

What Microsoft documents about the limits

Every figure below comes off Microsoft’s own documentation and was pulled on 4 September 2026. Two pages carry the lot of it, “About shared mailboxes in Microsoft 365” and “Exchange Online limits”, and they are worth going and reading your own self before you take anybody’s word for any of it, ours as much as anybody else’s.

Read that table and the temptation is to worry about the sending numbers, on account of them being the biggest ones on it. They are not what ends this arrangement. That is the shape of it. Two rows do the ending, which are the 25 and the 50 GB, and a third thing that is not in any table anywhere gets there before either of them, and that one is next.

Why your replies are not in the shared mailbox by default

Here is the sentence that costs desks the most, straight out of the Microsoft documentation. Messages sent from the shared mailbox are not saved to the Sent Items folder of the shared mailbox. They are saved to the Sent Items folder of the person who sent the message.

Sit with that a minute, seeing as the whole appeal of a shared mailbox is that everybody can see what happened on a thread. Out of the box they cannot. Nobody warns you either. The incoming half is shared and the outgoing half is scattered over four personal mailboxes, so when a customer rings on the Thursday asking why nobody answered them, the answer to that might be sitting in somebody’s own Sent Items, and that somebody is off until the week after next.

It is a switch and it takes about a minute to throw. Over in the Microsoft 365 admin centre the shared mailbox has a Sent items setting, and turning it on puts a copy of whatever went out as the mailbox into the mailbox’s own Sent Items where the rest of you can read it. Nobody is stopping you. The trouble is that nothing anywhere tells you the setting is there, so a desk finds out about it on the day it badly needed the record, and the mail from before that day stays scattered where it landed, no two ways about it.

Why 25 users is the ceiling that ends the argument

Every growing desk gets to the same question eventually, which is how many of us can be in this thing at once, and it is one of the few around here with a documented answer. A shared mailbox supports a maximum of 25 users. Where too many people are in it at the same time they might see connection failures or duplicated messages, and Microsoft’s own advice at that point is to go and use a Microsoft 365 group instead.

Duplicated messages on a support desk is not a small thing at all. It means the same customer mail turning up twice, which means two of you picking up what looks like two separate pieces of work, and the fix for that one is a conversation rather than a setting. Fair is fair, the most of us never get near 25 people in a mailbox at once, so this is a ceiling only the growing desks go and meet.

The one that arrives long before it is smaller and is written down nowhere by anybody, on account of it being a fact about people rather than about software. Somewhere around 6 or 8 of you the conventions stop holding. Two is fine. Two people talk to each other constantly and hardly need a system for it. Four is fine on a quiet week and shaky on a loud one, and by 8 the agreement about who takes what wants saying out loud several times a day, which stops happening properly on precisely the days it matters, those being the days everybody is busy.

What happens on the morning it fills up

The 50 GB feels enormous until you go and look at what a support mailbox actually carries. Every screenshot a customer sends, every invoice, every phone photograph of a broken thing, and every one of them stored twice over the minute a reply quotes the thread back underneath it. A desk taking a few hundred mails a week with attachments on a good share of them fills the thing in a few years rather than a few decades, day in, day out, and the filling of it is invisible the whole time it is happening. Nothing counts down.

Microsoft documents the sequence at the limit and it is worth knowing in the right order. First you keep receiving for a while and you cannot send. Then, after some time, it stops receiving as well, and people writing in get a non-delivery report back. Your customers meet it first.

Which is the part that turns a storage problem into a customer problem. Your customers are told the message could not be delivered, in the standard wording, by your own mail system, on behalf of the address printed on your website. Nobody on the desk gets an alert about it either. The first you know of it is somebody ringing to ask whether you have gone out of business, and by that hour it has been happening for however long it has been happening.

The fix is to put an Exchange Online Plan 2 licence on the mailbox, which lifts it to 100 GB. There is a wrinkle in it worth knowing as well: unlicensed shared mailboxes made before July 2018 already sit at 100 GB, so an old mailbox and a new one behave differently and neither of them tells you which one you have got. Moving the big attachments off into SharePoint buys you room, all the same, and none of it gets you a warning ahead of the event. Put a note in the calendar to look at the size twice a year, on account of you being the alerting system over here.

Why two of you answer the same email, and nothing in there minds

Nothing in a shared mailbox assigns anything to anybody. That is not a gap in the product, it is what the product is, and every desk running on one goes and invents a layer of agreement to sit on top of it. People sort it out among their own selves, in Teams or Slack or across a desk, and the mailbox never hears about any of it.

The agreements are all the same handful and they all fail in the same way. Categories get used as a status, coloured by person or by state, and they work grand until somebody forgets to set one or two people set different ones on the same thread. Folders get used as a queue, dragging a mail into “Being dealt with”, which works until a reply lands and the thread is now in two places. Read and unread gets used as a claim, which is the worst of the lot, since unread means nobody has opened it and also means somebody opened it and marked it back. Then there is the flag, which is shared, so your flag is everybody’s flag and nobody’s.

None of it is enforced. Not one of those conventions is a rule the mailbox knows the first thing about, and there is no report anywhere that will tell you who broke which. Nobody is minding it. The whole of it holds together because people are talking to each other, and it comes apart quietly on the week somebody is off sick, which is the very week you needed it holding.

You cannot stop the deleting either, and Microsoft says so plainly: you cannot prevent users from deleting messages in a shared mailbox, and if you need that, use a group instead. On a support desk a deleted mail is a customer nobody ever answered, and your undo is the Deleted Items folder plus 14 days after it is emptied, unless somebody has gone and raised that to 30.

Which jobs it cannot do at all, however you arrange it

Some of what follows can be worked around with enough discipline. These cannot, and no amount of Outlook rules gets you there.

There is no clock in it. Nothing measures how long a message waited, nothing warns you at 4 hours, and nothing tells you at the end of the week which ones sat longest of the lot. A promise about response times is a thing you can make and not a thing you can watch, and that is the whole difference between a target and an SLA policy. First response time is a number you simply have not got.

There is no report either. At the month end somebody asks how many came in, how long they waited and what they were about, and the honest answer out of a shared mailbox is a count of items in a folder, which answers the first question badly and the other two not at all. You can export to CSV and sit counting it by hand, mind you, and one afternoon spent doing that generally settles the wider argument on its own.

Then two documented ones that catch people out. Mail sent from a shared mailbox cannot be encrypted, on account of the mailbox having no security context of its own to hold a key with, which matters a good deal where you are working under HIPAA or answering a GDPR question about how the thing is handled. And access is for people inside your organisation only, so a contractor, an agency or an outsourced evening shift cannot be put on it at all, whatever the arrangement with them says.

Automatic replies are text, with no images allowed in them. Small thing. It is the sort of small thing that turns up on the afternoon somebody wanted the acknowledgement to carry the logo.

What to set up on day one, if you are staying on it

Six things, an hour between them, and they are the difference between a shared mailbox that holds up for two years and one that becomes a row in month four.

1. Turn on the Sent items copy in the Microsoft 365 admin centre, so replies sent as the mailbox land in the mailbox rather than in one person’s own Sent Items.
2. Check the sign-in on the shared mailbox account is blocked, and leave it blocked. Microsoft asks for this twice and it is the account nobody ever thinks about. The Email apps setting on the same mailbox decides whether IMAP and POP3 can reach it at all, and SMTP AUTH is its own separate switch again.
3. Agree one categories scheme, keep it to about 5 colours, and write it somewhere people will actually see it. Two schemes running at once is worse than none.
4. Raise the deleted-item retention from the default 14 days to 30, since it is the only undo you have and the change costs nothing.
5. Put one signature on the mailbox and give everybody the same one, otherwise the customer meets four different companies depending on who picked the mail up.
6. Put a reminder in the calendar twice a year to look at the mailbox size against the 50 GB, because nothing else is going to tell you.

Do the first one today even if you never get round to the rest of it. It is a minute of work, plainly said, and it is the only one on the list that quietly rewrites what the mailbox is for.

Where a shared mailbox is genuinely enough

A good many desks ought to stay put where they are, and we would say that to their faces.

Two or three people answering support, all of them within earshot or sitting in the very same Teams channel, a few dozen mails a week, nobody asking for a report at the month end and nothing landing that has to be answered inside a promised hour. Leave the thing alone. That desk gets everything it needs out of a shared mailbox and out of the licences it already pays Microsoft for. Buying a help desk for that desk, ours or anybody else’s, is money going out on a problem they have not got.

The honest test is not the size of the team either, it is whether the conventions are holding. Where nobody has answered the same mail twice this month, where nobody has had to ask what happened on a thread, and where the month end question has not been asked of you, the arrangement is working and there is nothing to fix.

Where it has stopped holding you will know it already, on account of the evidence being conversations rather than numbers. Somebody says sorry, I thought you had that one. Somebody cannot find what was promised. That is what outgrowing the thing sounds like, and it goes on sounding like that for months before anybody writes a word of it down. Over on Google Workspace the same story reads a little differently, funnily enough, and we have set that out separately in our piece on the Google Collaborative Inbox, which is built on Google Groups and stops in its own places for its own reasons.

What we make, and what it costs

Maxdesk is a help desk, and what it adds to the arrangement above is the layer the mailbox has not got. A ticket has an owner, a status and a priority, so who is on what is a fact rather than an agreement. There is a clock on it with a first reply and a resolve target, defaulting to 1 hour and 8 hours until you set your own. There are reports that answer the month end question without anybody counting a folder by hand. Automation rules tag and route and acknowledge without a person watching, and the knowledge base holds the answers you keep retyping.

Nobody here gets charged by the seat. The price is per workspace and it stays put whether four of you are on it or fourteen, with Free at $0 forever on unlimited agents and unlimited tickets under fair usage, Pro at $20 a month, Elite at $99, and the AI features living on Elite alone. Our free help desk software plan runs ads on it, stamps our name onto whatever goes out, and keeps 3 months of history behind it. A shared mailbox does none of those three things to you, which is a real point in Microsoft’s favour and not one we intend to pretend about.

None of this replaces Microsoft 365. Not a bit of it. Your mail still arrives where it arrives and your people keep Outlook for their own mail, and support@ forwards into the desk instead of being opened by four people at once. What that leaves you is the shared inbox behaviour with ownership attached to it, and if the decision itself is what you are stuck on rather than the mechanics, we have written the whole argument out under when a shared inbox becomes a ticketing system.

The parts of this we would argue with ourselves

The move costs a fortnight of small annoyances and we would sooner say that than sell around it. New habits, a new place to look, somebody still answering out of Outlook for the first while of it, and a week where the whole thing feels slower than it was. Where the shared mailbox is coping, that fortnight buys you nothing at all.

Mail is the only channel we ship, which is the very same limit the shared mailbox has. Anybody ringing you, or asking in person, leaves no trace in either system, so nothing on this page fixes the half of support that happens out loud.

We also have no migration tooling for this, said plainly. There is no button anywhere that lifts three years of shared mailbox history into a desk, ours or anybody else’s, and what most teams do is start fresh on a date and keep the old mailbox open and readable for a good long while afterwards. That is the honest answer to it and it is the one people are surprised by.

And the figures. Every Microsoft number on this page came off their documentation on 4 September 2026, from pages dated between February and April of this year, and vendors change these. Our own prices and plan details were checked the same day. We have pencilled a re-check in for early 2027. The sensible habit, if you are deciding something off the back of one of these numbers, is to open the Microsoft page and read the figure your own self before you act on any of it.

Where to start, if you are not sure which side of the line you are on

Open the shared mailbox and give it half an hour with last month’s mail. It will tell you more than another page of ours will.

Count the threads where two of you replied. Then go and look in the mailbox’s own Sent Items to see whether the replies are in there at all, seeing as that one check tells you inside a minute whether the setting further up this page was ever turned on. Right-click the mailbox after that, look at the folder size against the 50 GB, and work out roughly how long you have got at the rate the thing has been filling. Last of the lot, pick the five oldest open threads and ask how long each one sat before anybody answered it, and notice how long that took you.

If those four answers come back comfortable, you have your answer and it costs nothing. If two of them made you wince, the ceiling you are meeting is not one of Microsoft’s, it is the one about people, and no licence lifts that one. Ours is free to start on and the sending limits on your own mail are a separate question again, which we have covered for the Google side under Gmail sending limits, and the choice between the mailbox shapes themselves sits in our piece on the shared mailbox against a distribution list.

Common questions about Outlook shared mailboxes

Can you use an Outlook shared mailbox as a help desk?
Yes, and plenty of small teams do. It gives several people one address, costs nothing extra, and works well up to about 4 or 5 people at modest volume. It has no ticket ownership, no clock and no reporting, so the arrangement rests on conventions your team agrees and maintains by talking.

How many users can access an Outlook shared mailbox?
Microsoft documents a maximum of 25 users on one shared mailbox. Above that, people may see connection failures or duplicated messages, and Microsoft’s own advice is to use a Microsoft 365 group instead. In practice the conventions that decide who answers what tend to fail somewhere around 6 to 8 people, well before the technical ceiling.

How big can a shared mailbox be?
50 GB without any licence assigned to it, or 100 GB if you assign an Exchange Online Plan 2 licence. Unlicensed shared mailboxes created before July 2018 are already 100 GB. At the limit you can keep receiving for a while but cannot send, and after that senders get a non-delivery report.

Why can I not see replies sent from our shared mailbox?
By default Microsoft saves messages sent from a shared mailbox into the personal Sent Items of whoever sent them, not into the shared mailbox. Turn on the Sent items setting for that mailbox in the Microsoft 365 admin centre and copies land in the mailbox instead. Mail sent before you change it stays where it went.

Does a shared mailbox need a licence?
Not for ordinary use up to 50 GB. A licence is needed to go to 100 GB, to put the mailbox on litigation hold, or to give it a proper archive. Everyone accessing it does need their own licensed Exchange Online mailbox, so the shared mailbox is free but the people are not.

When should you move off a shared mailbox?
When two people start answering the same message, when somebody cannot find what was promised to a customer, or when a month-end question about volumes and waiting times cannot be answered without counting by hand. Those three arrive before any of Microsoft’s documented limits do.