Postfix + Cyrus + SASL AUTH mail server,

I have noticed that users can send mail in other users name just by setting
Outlook mail client email address and display name.
I have enable SASL AUTH but Postfix relays mails as any user so long as you
are authenticated.

I can send mail in any users name by just setting Outlook with any other
user's address and display name and authenticating with my login name and
password!

How can I stop this?
Please help.

Regards,

Clifford Gonsalves