Add shared mailbox paragraph (#474)

* Add shared mailbox paragraph

Explain shared and public folders
Document SharedSeen dovecot prop

* Apply suggestions from code review

Co-authored-by: Davide Principi <davide.principi@nethesis.it>

administrator-manual/en/mail.rst

@@ -270,6 +270,24 @@ use the following credentials:
 * user name: ``john*root``
 * password: ``secr3t``
 
+Users could share their mailbox (or some parts of it, folders) with selected accounts on the system. 
+Everyone who is given access to a shared mailbox can read or delete messages according to permissions
+granted by the mailbox owner.
+
+An IMAP flag named ``/Seen`` is used to mark if a message has been read or not. In a shared mailbox,
+each user has their copy of the messages they have read, but sometimes a team sharing a mailbox
+could prefer to know if a mail has already been read by someone else.
+To enable sharing of the ``/Seen`` flag for all shared mailboxes use the following commands: ::
+
+    config setprop dovecot SharedSeen enabled
+    signal-event nethserver-mail-server-save
+
+Please note that changing the ``SharedSeen`` status resets the ``/Seen`` flag for all users on all mailboxes.
+
+Public folders are created by the administrator and are usually visible to all users (or large groups).
+The ``/Seen`` flag is kept for each user and it cannot be shared.
+
+
 .. _email_messages:
 
 Messages