The main difference between a user mailbox and a shared mailbox is that the shared mailbox is associated with a disabled user. A shared mailbox is mainly used to allow login access for multiple users. It is the same when multiple users use a user mailbox and a shared mailbox at the same time.
A user mailbox can work as a shared mailbox, as long as you grant full mailbox access permission on the user mailbox to the other users. These users can log onto the user mailbox. However, it is not recommended. For security concerns, if you want multiple users to log on to one mailbox, please use a shared mailbox.
User Mailboxes
A User mailbox is usually associated with a single user and requires a license. A mailbox is required for a user to log on to and use Office 365 email.
Mailboxes require a paid license
Users need a mailbox in order to log in and use email
Shared Mailboxes
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 are not supported on mobile devices
Shared mailboxes appear as separate mailboxes in Outlook and Outlook on the web.
A user can send an 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
The Difference
If a user replies to a message in a shared mailbox, the reply appears to come from the shared mailbox email address.
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.
A regular mailbox by default goes to the user.
You can't log into a shared mailbox, so it can't be the Outlook profile mailbox, but it can be added as an additional mailbox.
You cannot set a shared mailbox as a primary profile in Outlook.
Anyone with permission to the shared mailbox can access it with both Outlook and OWA. If the user also has Send As permission, they can select the mailbox name for the To: field when they send the message.
You can add the -Automapping $True switch when you grant the user permission to the mailbox.