A shared mailbox is just that, a mailbox that can be shared with one or more users. Shared mailboxes do not require a license and have all of the features of a normal mailbox; they have an inbox, a calendar, a contact list etc.
Shared mailboxes cannot be logged into directly, a user must have their own mailbox to be able to use a shared mailbox
Shared mailboxes are not supported on mobile devices
Shared mailboxes appear as separate mailboxes in Outlook and Outlook on the web.
A user can send email and have the message appear to come from the shared mailbox email address
If a user replies to a message in a shared mailbox, the reply appears to come from the shared mailbox email address
A user can also (if they have permission to do so) send a message “on behalf of” a shared mailbox. The message will appear to come from “Jane Doe on behalf of <Shared Mailbox>”
Calendars and contact lists in shared mailboxes are fully useable by users having access to the shared mailbox
Email sent from a shared mailbox do not by default appear in the sent items of the shared mailbox but rather the sent items of the user sending the email message. This behavior can be changed however
Shared mailboxes support subfolders i.e. it is possible to create folders under inbox for example in order to organize email