Both a shared mailbox and a user mailbox live in Exchange Online and both send and receive mail, so on the surface they look interchangeable. They aren’t. The difference comes down to who signs in, who pays for a license, and how much storage and feature coverage you get. Pick the wrong one and you either burn licenses you didn’t need or hit a wall when a team address suddenly needs to be a real account.
This guide lays out exactly where the two differ, when each one is the right call, and how to convert between them without losing mail. If you only remember one thing: a user mailbox is for a person who signs in, and a shared mailbox is for an address several people work from.
The short version
A user mailbox belongs to one identity that can sign in to Microsoft 365, and it requires a license that includes Exchange Online. A shared mailbox is a mailbox with sign-in disabled that several licensed users open through delegated permissions, and it doesn’t need its own license until it crosses 50 GB or needs specific compliance features.
Shared mailbox vs user mailbox at a glance
| Interactive sign-in | Shared: disabled by default | User: yes |
|---|---|
| License required | Shared: no, under 50 GB | User: yes (plan with Exchange Online) |
| Storage limit | Shared: 50 GB without a license | User: 50–100 GB depending on plan |
| Online archive | Shared: needs a license | User: included on most plans |
| Owned by | Shared: a team | User: one person |
| Typical use | Shared: info@, support@, sales@ | User: a named employee |
What a user mailbox is
A user mailbox is the normal mailbox you get when you create a licensed account. It maps to one person, that person signs in with their own credentials, and the mailbox carries their calendar, contacts, and Teams identity. Because it’s tied to a licensed account, it gets the full storage and feature set of whatever plan you assigned — typically 50 GB on lower plans and up to 100 GB on higher ones, plus an online archive.
Use a user mailbox whenever an address belongs to a single human who needs to log in, manage their own mail, and show up as themselves in Teams, SharePoint, and the rest of Microsoft 365.
What a shared mailbox is
A shared mailbox is built for an address that a group works from rather than a single person. It still has a real SMTP address and a real mailbox, but its account sign-in is disabled, so nobody logs in as the mailbox. Instead, you grant your team members Full Access and Send As, and they open it through their own accounts.
The headline benefit is cost. A shared mailbox doesn’t need its own license for ordinary use, so a five-person support desk can share support@ without paying for a sixth account. When someone leaves the team, you remove their access instead of resetting passwords or untangling a personal account. If you haven’t set one up yet, the steps are in how to create a shared mailbox in Microsoft 365.
Licensing: where the money is
This is the difference that matters most to anyone watching the bill. A user mailbox always needs a license. A shared mailbox is free for normal use, with specific exceptions.
You need to assign a license to a shared mailbox when:
- It grows beyond 50 GB. Exchange Online stops delivering to an oversized unlicensed shared mailbox.
- You want to enable an online archive for it.
- You apply litigation hold or certain retention features that require a license.
Everything else about a shared mailbox is license-free, including the address, basic mail flow, and delegated access. The catch people miss: every person who opens the shared mailbox still needs their own licensed account. The mailbox being free doesn’t make the users free.
When each mailbox type needs a license
| User mailbox | Always — assign a plan with Exchange Online |
|---|---|
| Shared mailbox under 50 GB | No license needed |
| Shared mailbox over 50 GB | License required to keep receiving mail |
| Shared mailbox with archive | License required |
| Shared mailbox on litigation hold | License required |
If you’re trying to right-size who gets what plan, assigning Microsoft 365 licenses covers the mechanics of attaching and swapping licenses cleanly.
Sending mail: the behavior that confuses users
With a user mailbox, sending is simple: the person signs in and sends as themselves. A shared mailbox is where the support tickets come from, because Full Access and Send As are separate permissions.
- Full Access lets a user open the shared mailbox and read, move, and delete items. On its own it does not let them send as the mailbox.
- Send As lets a user send so the message looks like it came straight from the shared address.
- Send on Behalf sends as “User on behalf of the mailbox,” so the recipient sees who actually replied.
For a normal team address, grant Full Access plus Send As. If a user replies and the message goes out from their personal address instead of the shared one, that’s the classic sign they have Full Access but not Send As.
Connect-ExchangeOnline
# Full Access (auto-maps the mailbox into Outlook)
Add-MailboxPermission -Identity [email protected] `
-User [email protected] -AccessRights FullAccess -InheritanceType All
# Send As so replies carry the shared address
Add-RecipientPermission -Identity [email protected] `
-Trustee [email protected] -AccessRights SendAs -Confirm:$falseBy default, mail a user sends as the shared mailbox is copied into the shared Sent Items, so the whole team can see what went out. If that isn’t happening, check the per-mailbox setting:
Set-Mailbox -Identity [email protected] -MessageCopyForSentAsEnabled $trueWhen to choose which
Most decisions are clear once you ask who owns the address and whether anyone needs to sign in.
Choose a shared mailbox when
- Several people work the same address (info@, support@, sales@)
- Nobody needs to sign in directly as that address
- You want a shared inbox and shared Sent Items history
- You want to avoid paying for an extra licensed account
- Total storage will stay under 50 GB
Choose a user mailbox when
- One named person owns the address
- That person needs to sign in and use Teams, OneDrive, and SharePoint as themselves
- You need more than 50 GB or an online archive
- The mailbox needs its own Conditional Access or device policies tied to a sign-in
A common middle case is a role address that one person currently handles but a team will inherit later — like a small company’s accounts@ run by one bookkeeper. Start it as whatever matches today’s reality, because converting later is straightforward.
Converting between the two
The most common move is converting a user mailbox to a shared mailbox when someone leaves. You keep their mail accessible to the team and reclaim the license.
In the Microsoft 365 admin center, open the user under Users → Active users, go to the Mail tab, and choose Convert to shared mailbox. After it converts, remove the license. With PowerShell:
Connect-ExchangeOnline
# Convert the leaver's mailbox to shared
Set-Mailbox -Identity [email protected] -Type Shared
# Then remove the license from the account in the admin centerConverting a shared mailbox back to a user mailbox is the same idea in reverse: assign a license, then set the type to Regular, then re-enable sign-in if a person will own it.
# After assigning a license to the account
Set-Mailbox -Identity [email protected] -Type RegularQuick gotchas worth knowing
A few details trip up admins who treat the two mailbox types as identical:
- A shared mailbox can hold a calendar and contacts, so a small team can share a calendar without a Microsoft 365 group. It just won’t carry a Teams or SharePoint identity.
- Auto-mapping adds a shared mailbox to Outlook automatically with Full Access, but it can take time and may need an Outlook restart. Send As and Send on Behalf never auto-map anything.
- If you need a leaver’s mail to forward somewhere while the team transitions, set forwarding on the converted shared mailbox — see set up email forwarding in Microsoft 365.
Bottom line
Reach for a user mailbox when an address belongs to one person who signs in and needs the full plan. Reach for a shared mailbox when several people work the same address and nobody needs to log in as it — and enjoy the license you didn’t spend. Keep shared mailboxes under 50 GB to stay license-free, remember that the people accessing them still need their own accounts, and convert in the right order when staff change.
