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A shared mailbox is a mailbox that multiple users can use to read and send email messages. Shared mailboxes can also be used to provide a common calendar, allowing multiple users to schedule and view vacation time or work shifts.
Why set up a shared mailbox?
Provides a generic email address (for example, email@example.com or firstname.lastname@example.org), that customers can use to inquire about your company.
Allows departments that provide centralized services to employees (for example, help desk, human resources, or printing services), to respond to employee questions.
Allows multiple users to monitor and reply to email sent to an email address (for example, an address used specifically by the help desk).
A shared mailbox is a type of user mailbox that doesn't have its own user name and password. As a result, users can't log into them directly. To access a shared mailbox, users must first be granted Send As or Full Access permissions to the mailbox. Once that's done, users sign into their own mailboxes and then access the shared mailbox by adding it to their Outlook profile. In Exchange 2003 and earlier, shared mailboxes were just a regular mailbox to which an administrator could grant delegate access. Beginning in Exchange 2007, shared mailboxes became their own recipient type:
In previous version of Exchange, creating a shared mailbox was a multi-step process in which you had to use the Exchange Management Shell to complete some of the tasks. In Exchange 2013 and later, you can use the Exchange admin center (EAC) to create a shared mailbox in one step. For details, see Create shared mailboxes in the Exchange admin center. In fact, the EAC has a feature area devoted entirely to shared mailboxes. Just navigate to Recipients > Shared mailboxes to view all the management tasks for shared mailboxes.
You can use the following permissions with a shared mailbox.
Full Access: The Full Access permission lets a user log into the shared mailbox and act as the owner of that mailbox. While logged in, the user can create calendar items; read, view, delete, and change email messages; create tasks and calendar contacts. However, a user with Full Access permission can't send email from the shared mailbox unless they also have Send As or Send on Behalf permission.
Send As: The Send As permission lets a user impersonate the shared mailbox when sending mail. For example, if Kweku logs into the shared mailbox Marketing Department and sends an email, it will look like the Marketing Department sent the email.
Send on Behalf: The Send on Behalf permission lets a user send email on behalf of the shared mailbox. For example, if John logs into the shared mailbox Reception Building 32 and sends an email, it look like the mail was sent by "John on behalf of Reception Building 32". You can't use the EAC to grant Send on Behalf permissions, you must use Set-Mailbox cmdlet with the GrantSendonBehalf parameter.
Link to Article on Microsoft: https://docs.microsoft.com/en-us/Exchange/collaboration/shared-mailboxes/shared-mailboxes?view=exchserver-2019