An email address doesn't need a person behind it. But walk through most Microsoft 365 tenants and you'll find info@, careers@, billing@, events@ — even alert@ — each parked on a licensed account. Every one of those is a mailbox license with nobody sitting in the chair.
Microsoft's rule is refreshingly simple: a shared mailbox stores up to 50 GB with no license at all, and there's no cap on how many you can create. The people opening it still need their own Exchange Online license — the mailbox is free; the readers aren't.
The math
| Mailbox / measure | Amount |
|---|---|
| Exchange Online Plan 1 | $4/mo |
| Per generic inbox per year | $48 |
| The same inbox on a full E3 seat | $39/mo = $468/year |
| Shared mailbox (up to 50GB) | $0 |
| Six generic inboxes converted off Plan 1 | $288/year back |
| Six converted off full E3 seats | $2,808/year back |
The decision rule, per inbox
- Address belongs to no person and sends/receives → shared mailbox, $0.
- Mailbox already over 50GB → license it (Exchange Online Plan 2 at $8/mo raises the cap to 100GB).
- Litigation hold, in-place archive, or retention policies on it → license it appropriately (Plan 2, or Plan 1 plus the Exchange Online Archiving add-on).
- Someone signs in to it directly → that's a user mailbox and it needs a user license — keep access delegate-only.
Step by step
- Find the ghost seats. In the admin center, go to Billing → Licenses and list every account with no real person behind it (info@, careers@, billing@, events@, noreply@, alert@).
- Size each mailbox. Anything under 50GB converts cleanly; over 50GB, price Plan 2 or slim the mailbox first.
- Create the shared mailbox (or convert the licensed account — Microsoft's own recommended pattern for leavers' mailboxes too).
- Grant Full Access and Send As to the people who work the inbox.
- Keep it delegate-only. Microsoft blocks direct sign-in on shared mailboxes by default; leave it blocked.
- Remove the license from the old account and verify the seat count drops at renewal.
Six generic inboxes moved off Exchange Plan 1 licenses = $288/yr back ($24/mo); parked on full E3 seats = $2,808/yr.
In the admin center.
Prices verified October 20, 2026: Exchange Online Plan 1 ($4) and Plan 2 ($8) per user/month, annual commitment, on Microsoft's Exchange plans pricing page. Shared mailbox rules (50GB free without a license; Plan 2 for 100GB; Plan 2 or Plan 1 + archiving add-on for litigation hold/in-place archive; no direct sign-in; accessing users need their own license) from Microsoft Learn 'About shared mailboxes in Microsoft 365'.
Editorial note: The two traps to avoid: (1) converting a mailbox that's under litigation hold or a retention policy — license it instead and move deliberately; a license is cheap, losing the hold is not. (2) Letting someone log straight into the shared account — that's direct sign-in, which makes it a user mailbox again and it needs a license. Delegate access only.
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