SBX Tools Do Shared Mailboxes Need a Microsoft 365 License? Costs, the 50 GB Limit, and Exceptions
Short answer: a shared mailbox does not need its own Microsoft 365 license as long as it stays under 50 GB and you don't enable an online archive or litigation hold. Cross any of those thresholds and you must assign it a license — typically an Exchange Online Plan 2. Separately, every person who opens the shared mailbox still needs their own Exchange Online license.
When a shared mailbox is free
A standard shared mailbox is included with Exchange Online at no extra cost. You create it, grant your team Full Access and Send As permissions, and everyone works from it without buying anything for the mailbox itself. This covers the vast majority of role-based inboxes like support@, info@, sales@, and finance@. The mailbox has no sign-in of its own — nobody logs in with shared credentials; access comes entirely from the permissions you assign to already-licensed users.
When a shared mailbox does need a license
There are three main exceptions where Microsoft requires a license on the shared mailbox:
| Situation | What's required |
|---|---|
| Storage grows beyond 50 GB | An Exchange Online Plan 2 license (raises the limit to 100 GB) |
| You enable an online (in-place) archive | A license that includes archiving (Exchange Online Plan 1 with archiving, or Plan 2) |
| You place it on litigation hold | An Exchange Online Plan 2 license (or Plan 1 with the compliance add-on) |
If none of these apply, leave the mailbox unlicensed.
Do the people using the mailbox need licenses?
Yes — but they almost certainly already have them. Every user who accesses a shared mailbox must have their own Exchange Online license (usually via Microsoft 365 Business or Enterprise). The saving is that the shared mailbox is free; you don't buy an extra seat just because five people all read support@.
What about the 50 GB limit in practice?
A busy shared inbox with years of history and large attachments can approach 50 GB over time. Before it does, you can archive or clean up old mail, or assign an Exchange Online Plan 2 license to lift the ceiling to 100 GB. It's worth monitoring, because if a shared mailbox hits its limit it can stop receiving new mail.
SBX doesn't change any of this — it runs on top of your existing shared mailbox or Microsoft 365 Group and doesn't alter your Microsoft licensing. SBX stores only metadata (assignee, status, timestamps), never the content of your emails, so your mailbox size and Microsoft license requirements are exactly what they'd be without SBX.
Frequently asked questions
Does a shared mailbox require a Microsoft 365 license?
What is the storage limit for an unlicensed shared mailbox?
Do users need a license to access a shared mailbox?
Can I sign in to a shared mailbox directly?
Does adding SBX change my licensing?
For Microsoft's official guidance, see About shared mailboxes and, to choose the right address type, our comparison of Microsoft 365 Group vs shared mailbox vs distribution list.