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A shared mailbox is a mailbox that multiple users can open to read and send e-mail messages. Shared mailboxes allow a group of users to view and send e-mail from a common mailbox. They also allow users to share a common calendar, so they can schedule and view vacation time or work shifts. You can't log into a shared mailbox directly using Outlook or Outlook Web App. You must first be granted permissions to the shared mailbox, and then you access it using Outlook or Outlook Web App. You don’t need to assign licenses to shared mailboxes, except when they are over their storage quota of 10 gigabytes (GB).
If you're using Office 365 Enterprise, Business Premium or Business Essentials, check out Create and use shared mailboxes in Office 365 for the steps.
If you're using Office 365 Small Business, check out Create and use shared mailboxes in Office 365 for Small Business.