Syncronice mechanism for mail recived from diferents location or u

[Double]

In a mail account configured to preserve the mails on serverMail for
syncronice with others users or computers. What mechanism uses outlook &
mailServer for determine that the email account instance connected is an
diferent computer or user, to re-send the emails???.

I need to use the same email account on the three win XP users from a same
PC, but the each time I received mail, the server is re-send a copy for each
user. I want a copy only for the computer, because the users are sharing the
..PST files. (and the eMails are yet present only with one connection. On
second connection we don't want to obtain again the same mail).

I hope that crossing the syncronice mecanish we can solve...

Thanks