A Microsoft 365 shared mailbox is free, and the price of free is a set of limits that Microsoft documents in one place and teams discover one at a time. This guide gathers the limits, explains when a license becomes necessary, and then turns to the part no Microsoft page covers: how a team of five should run the thing.
A shared mailbox needs no license up to 50 GB, supports a maximum of 25 users, has sign-in blocked, and takes internal members only. Going to 100 GB, adding an archive or placing it on litigation hold needs Exchange Online Plan 2, or Plan 1 plus Exchange Online Archiving. For the team: one owner per thread, categories as states, a scheduled triage pass, and the Sent items copy setting turned on.
What are the limits of a Microsoft 365 shared mailbox?
| Limit | Value | What it means for a team |
|---|---|---|
| Storage without a license | 50 GB | At the cap, receiving continues for a while but sending stops; later, senders get bounces |
| Storage with Exchange Online Plan 2 | 100 GB | The only way to grow the primary mailbox |
| Archive | Needs Plan 2, or Plan 1 plus Exchange Online Archiving | Either license also allows auto-expanding archiving |
| Users | Maximum 25 | Beyond that Microsoft warns of connection failures and duplicated messages, and points to a Microsoft 365 group |
| Direct sign-in | Blocked by default | Members open it from their own licensed mailbox |
| External members | Not allowed | A Microsoft 365 group can hold guests; a shared mailbox cannot |
| Encryption | Not available from the shared address | The mailbox has no security context of its own, so no encryption key |
| Deletion | Cannot be prevented | Any Full Access member can delete; items moved between folders are kept in Recoverable Items |
| Inbox rules | 256 KB of rules per mailbox | Same cap as a user mailbox; a migrated mailbox may start lower and can be raised |
| Mailboxes created before July 2018 | 100 GB | Older unlicensed shared mailboxes keep the larger size |
Sources: About shared mailboxes in Microsoft 365 and Exchange Online limits.
Does a shared mailbox need a license?
Not by default. The shared mailbox itself is unlicensed for up to 50 GB. Every person who opens it needs their own licensed Exchange Online mailbox. Your subscription must include Exchange Online at all; Microsoft 365 Apps for business does not, Business Standard does.
A license on the shared mailbox becomes necessary in four cases, all from Microsoft's licensing notes.
- The mailbox will exceed 50 GB. An Exchange Online Plan 2 license raises the cap to 100 GB.
- You want an archive mailbox. Plan 2, or Plan 1 with the Exchange Online Archiving add-on, enables it and allows auto-expanding archiving.
- Legal needs a litigation hold on the mailbox. Same licenses as the archive case.
- You want Defender for Office 365, Purview eDiscovery (Premium) or retention policies applied to it. Those features may need the mailbox licensed for them.
The license is assigned to the shared mailbox's user account under Users > Active users, since licenses are user settings. Microsoft also states that an archive mailbox may only hold mail for the single entity it is licensed to. Using a shared mailbox as a Cc target to archive other people's mail is prohibited in the service description.
Law and accounting firms meet the hold and retention cases first. Sektra for legal and accounting describes how a shared client address works alongside those controls.
Shared mailbox vs user mailbox
A user mailbox belongs to one licensed person who signs in to it. It holds 50 or 100 GB depending on plan, and can encrypt mail and own an archive within that plan. A shared mailbox has no usable sign-in, is capped at 50 GB unlicensed, cannot encrypt, and exists to be opened by several people at once.
Converting a departing employee's user mailbox into a shared mailbox keeps the history and, under 50 GB, frees the license. How to create a shared mailbox in Microsoft 365 covers the conversion and the two things to do right after it.
Shared mailbox permissions
Three permissions govern a shared mailbox. Full Access opens it and lets a member read, file and delete. Send As sends mail that appears to come from the mailbox. Send on Behalf sends mail marked on behalf of the mailbox. A member with Full Access alone can read everything and send nothing.
Admins set the first two under Teams & groups > Shared mailboxes > Manage mailbox permissions in the Microsoft 365 admin center. The Exchange admin center has the same under Recipients > Mailboxes > Manage mailbox delegation. Send on Behalf uses Set-Mailbox with GrantSendOnBehalfTo.
Grant Full Access to people by name, not through a security group, if you want the mailbox to appear in Outlook on its own. Automapping is written to the member's mailbox at grant time and does not fire for group membership. See Create a shared mailbox. Details of Send As and Send on Behalf, and the Sent Items consequence, are in how to send email from a shared mailbox.
Shared mailbox rules, automatic replies and forwarding
Inbox rules for a shared mailbox are created from inside the shared mailbox, and the practical way in is Outlook on the web. Select your name on the navigation bar, choose Open another mailbox, enter the shared address and select Open. In the window that opens, go to Settings > Mail > Rules. A rule created from your own mailbox's settings is stored in your mailbox and only ever sees your mail.
Admins can also create rules with Exchange Online PowerShell, using New-InboxRule with the -Mailbox parameter set to the shared address. See the New-InboxRule reference. Rules created this way run on the server, so they fire whether or not anyone has Outlook open.
Keep the total under the 256 KB cap. Outlook on the web offers inbox rules only; anything acting on outgoing mail is a mail flow rule in the Exchange admin center.
Automatic replies and forwarding are admin settings. In the Microsoft 365 admin center, open the shared mailbox under Teams & groups > Shared mailboxes and select Edit under Automatic replies or Email forwarding. Replies are text only, no images, and forwarding to several addresses needs a distribution list as the target. Both are on Configure shared mailbox settings. The same page has the Sent items setting that practice five below depends on.
What are the best practices for a shared mailbox?
Microsoft's documentation stops at configuration. The practices below are about the team, and they cost nothing to adopt.
- One owner per thread. The first person to open a message claims it by applying a category with their name. Nobody replies to a thread carrying someone else's category without a word. The mailbox has no owner field, so the convention is the system.
- Categories as states, and only a few. New, Waiting on customer, Waiting on us, Done. Write the four definitions in a pinned note. Five people with five private meanings for a flag is how a thread goes unanswered for a week.
- A triage pass at fixed times. Twice a day, one person sorts everything uncategorized, assigns owners and flags anything older than a day. Rotate the duty weekly.
- Reply from the shared address every time, with Send As, so the customer has one identity to answer and the thread stays in one mailbox.
- Turn on the Sent items copy setting the day the mailbox is created. Without it, half the conversation lives in personal folders.
- Grant permissions by name and keep the member list under 25. Prune leavers the same week they leave; a former employee's Full Access does not expire on its own.
- Watch the size. At 40 GB decide between a Plan 2 license, an archive, or a retention policy that moves old mail out. Discovering the cap when sending stops is the expensive version of this decision.
- Set an automatic reply that states a response time, then measure against it. The reply sets the customer's expectation; the triage pass is what meets it.
- Block sign-in and leave it blocked. New shared mailboxes are created that way; a converted user mailbox keeps its old password working until you reset it or block the account.
Where a shared mailbox stops
Run with those practices, a shared mailbox carries a small team a long way. It is built for a reception desk or a support address, and within that scope it works. What it never does is enforce any of the practices.
Ownership is a category someone may forget to apply. Waiting on us is a label, so nobody can see how many threads carry it or how old the oldest is. Each setting above is a per-mailbox change for an admin. And what we told this client in March is a search across the shared Sent Items and three personal ones.
We build Sektra, a separate shared inbox workspace that syncs two-way with Microsoft 365, including shared mailboxes. The mailbox stays in Microsoft 365 and the practices above become fields. Every thread has an owner and internal notes. A typing indicator reads 'A teammate is typing a reply'. The morning digest's Waiting on them list names the contacts who owe a reply.
A silence watch is a day count you set per contact, after which Sektra tells you the day they go quiet. It replaces the sticky note. Outlook connects through Microsoft sign-in, and Sektra never sees that password.
The Outlook integration page and the connect an Outlook shared mailbox article cover setup. Where an Outlook shared mailbox stops and a shared inbox tool starts covers the decision. The product page has the full feature list.
Keep the mailbox as long as the conventions hold. The limits above tell you when storage or headcount forces a change; the dropped threads tell you when the team does.
