What a shared mailbox is for
A shared mailbox is a mailbox that several people work from, rather than one person. Classic uses are info@, accounts@, sales@ and support@: mail arrives in one place, the team can see who has dealt with what, and replies go out from the shared address rather than an individual’s.
People with permission can open it alongside their own mailbox in Outlook, on the web or on the Outlook mobile apps, and can send as the mailbox or on its behalf. It also has its own calendar, which makes it useful for a team rota.
When it is free in Microsoft 365
In Microsoft 365, a shared mailbox can store up to 50 GB without being assigned a licence. For the overwhelming majority of team addresses that is plenty, and it is why shared mailboxes are the standard answer to “we need an info@ address” — rather than creating a fake user and paying for a licence.
One historical quirk: unlicensed shared mailboxes created before July 2018 were provisioned at 100 GB, so older tenants may see a mix of sizes.
The four situations that need a licence
- It grows past 50 GB. To raise the limit to 100 GB, Microsoft requires an Exchange Online Plan 2 licence on the shared mailbox. If you do nothing, the mailbox stops being able to send once it is full and eventually stops receiving, with senders getting a non-delivery report.
- It needs an in-place archive. Enlarging the archive needs Exchange Online Plan 2, or Plan 1 plus the Exchange Online Archiving add-on.
- It goes on litigation hold. Same requirement: Plan 2, or Plan 1 with Exchange Online Archiving. Firms with legal or regulatory retention obligations hit this one more often than they expect.
- You apply advanced security or compliance features — such as Defender for Office 365 or some Purview retention and eDiscovery capabilities — which can carry their own licensing requirements.
The rule everyone forgets: the people using it need licences
The shared mailbox itself may be free, but every person who opens it must have their own licensed mailbox. Shared mailboxes are reached through a user’s own mailbox, not by signing in to the shared one directly. You cannot use a shared mailbox to give mailbox access to someone who does not otherwise have one — and you cannot give access to people outside your organisation at all.
Microsoft also recommends that sign-in for the shared mailbox’s underlying account is blocked and kept blocked. It has a system-generated password that nobody is meant to know, and an account that can be signed into directly is an account that can be phished.
Other limits worth knowing
- 25 users. Microsoft states a shared mailbox supports a maximum of 25 users; beyond that, people can see connection failures or duplicated messages. Larger teams are better served by a Microsoft 365 group.
- No message encryption. Email sent from a shared mailbox cannot be encrypted, because the mailbox has no security context of its own.
- Anyone with access can delete. You cannot stop members deleting messages from a shared mailbox.
The leaver trick: keep the mail, drop the licence
When someone leaves, the usual question is what to do with their mailbox. Deleting it loses the history; leaving it as a licensed user means paying for someone who no longer works for you.
The answer is to convert it to a shared mailbox, give their manager access, and then remove the licence. Mail keeps arriving, nothing is lost, and — providing it is under 50 GB and not on litigation hold — it costs nothing. With prices rising in July 2026, this is one of the quickest savings available; our price rise guide lists the others.
How it works on hosted Exchange
On MyExchangeServer, shared mailboxes, room mailboxes and equipment mailboxes are free and unlimited — they never count towards your licensed mailbox total, and your own team can create them in the SolidCP control panel without raising a ticket. As with Microsoft 365, the people who use them need a mailbox of their own; at £8.00 + VAT per month that mailbox includes 50 GB of storage.