Postfix Configuration "Change of Sender Address"

[abid.h...@seecs.edu.pk]

Dear All,
I have a question regarding postfix configuration.

I have a mail server configured via postfix on centos. If an authorize user send email through command prompt he is able to change "from address/sender address" and can send email. I want to block this capability. i am explaining the problem point wise as below.

I have an authorized e-mail user by the name a...@xyz.com
It logs on at mail server through console by providing his credentials (uesrname/password)
after authentication he create a mail and change sender address from a...@xyz.com to k...@xyz.com and sends the mail to another legitimate user n...@xyz.com
In n...@xyz.com inbox the mail appears to be came from k...@xyz.com instead of actual sender a...@xyz.com
Kindly advise how change of "fro/sender address" can be blocked.

Regards