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, 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. In this case automapping connect the shared mailbox with the related users.
If you want share the mailbox in read only, or better with "reviewer" permissions, you have to remove the user from Full Access permissions and then use PowerShell to set the correct permissions.
Note: In this case the automapping will do not work
As a first step, check it out the default permission
Add-MailboxPermission -Identity SharedMailbox -User 'Username' -AccessRights ReadPermission
and then is time to set the "Reviewer" permissions for any folder you want to share in read only
Add-MailboxFolderPermission -Identity TSM:\Inbox -User firstname.lastname@example.org -AccessRights Reviewer
Add-MailboxFolderPermission -Identity TSM:\Outbox -User email@example.com -AccessRights Reviewer
In which TSM is the shared folder name
To remove permission, i.e.
Remove-MailboxFolderPermission -Identity TSM:\Outbox -User firstname.lastname@example.org
and the last but not the least, if you need it, you can setup the shared mailbox to copy sent items in your "sent items" folder
Set-Mailbox TSM MessageCopyForSentAsEnabled $True -MessageCopyForSendOnBehalfEnabled $True