Simple definition
A Shared Mailbox is an Exchange Online mailbox used by multiple people, for example support@, contact@, or reception@.
Technical definition
Users normally access the shared mailbox with their own accounts and assigned permissions. Full Access lets a user open and manage the mailbox; Send As lets them send messages that appear to come directly from the shared mailbox; Send on Behalf sends on behalf of it. A shared mailbox can also provide a shared calendar.
What is it used for?
Centralize a team or service address without sharing one password while controlling who can read the mailbox and who can send from it.
Practical example
Three support agents receive Full Access and Send As permissions on support@company.com. They work from the same mailbox in Outlook and reply using the support address.
Common issues
- Full Access is granted but Send As is missing
- Delegation was added but has not appeared in the client yet
- User tries to sign in directly as the shared mailbox instead of using delegation
- Rules, archive, or capacity require additional configuration or licensing
Key takeaway: Shared mailbox access is permission-based: Full Access and send permissions are separate and should be checked independently.