mailbox with more than one user behind it (but not mailing list)

There are several situations where mails go to mailboxes with more
than one user behind them, even thoug these are not simply distributing
mailing lists. The typical situations I know are:

- A boss and her depute/secretary both looking at the bosses email
   and answering it (sometimes the boss on her behalf, sometimes the
   secretary without saying so, sometimes the secretary saying so).

- An address for a help desk or something similar, where the volume
   is very big and/or constant replies are important so that more than
   one person is actually behind this address.

Requirements for these situations would typically be:

- Users want to trace that all incoming mails have been answered or
   have explicitly been flagged as not needing an answer.

- Several human users want to use the same mailbox, with all or more
   than one of them looking at all mail.

- Several human users want to use the same mailbox, with at least
   one of them looking at each mail.

- Individual users of a combined mailbox want to be able to reply in
   person (while still having the re-reply go to the mailbox) or on
   behalf of the mailbox.


Regards,   Martin.