Shared Mailbox

A shared mailbox is an Exchange Online mailbox that multiple users can read and send from, without a Microsoft 365 licence per user (up to 50 GB; beyond that, a licence is required). Users can't sign into a shared mailbox directly — they're delegated access via Full Access and Send As or Send on Behalf permissions, after which the mailbox appears as a separate folder tree in their Outlook. Used for team addresses like support@, sales@, or info@, where many people need to see incoming mail and respond from the shared identity. Mail to a shared mailbox does not fan out — it lands once in the shared inbox.

Send As vs Send on Behalf

The two send permissions look similar but produce a different From: address, and the difference matters to recipients. Send As makes the outgoing mail appear to come directly from the shared mailbox — support@contoso.com, with no trace of which team member actually typed it. Send on Behalf shows both: "Alice on behalf of support@contoso.com." Most support and sales scenarios want Send As, because customers shouldn't have to care which staff member replied; internal approval workflows sometimes prefer Send on Behalf specifically because the accountability is visible. Full Access alone grants neither send permission — it only lets a delegate open and read the mailbox — so a common configuration mistake is granting Full Access and assuming the user can also send, then being surprised when Outlook shows the mailbox but Send As silently fails.

Worked example

A five-person support team needs everyone to see and respond to support@contoso.com. An admin creates a shared mailbox, grants all five users Full Access and Send As permission (via the Exchange admin center or PowerShell), and each user's Outlook automatically shows the shared mailbox as an additional folder tree after the next sync — no separate sign-in, no separate password. A customer email lands once in the shared inbox; whichever team member opens and replies to it sends from support@contoso.com with no indication of which individual answered. When someone leaves the team, removing their access is a single permission change on the mailbox — nothing to migrate, because the mailbox and its history belong to the team, not to any one person's account.

Common pitfalls

The most common mistake is trying to sign into a shared mailbox directly with its own credentials — shared mailboxes have no usable password by design (a blocked, disabled account underneath), and that's deliberate: access should always flow through delegation, which keeps a clean audit trail of who actually did what. The second is letting a shared mailbox grow past the size where a licence becomes required and only noticing when mail delivery to it starts failing — worth a periodic check on mailboxes that have been running for years and accumulated attachments. The third is confusing a shared mailbox with a distribution list: a shared mailbox is one inbox everyone reads from a shared place, while a distribution list fans a copy out to every member's own inbox — teams that actually want everyone to have their own copy to triage independently usually want a distribution list or a Microsoft 365 Group, not a shared mailbox.