Originally Posted by aliabbass
you will be receiving and sending on the behalf of that person rather you will be using his/her account.
I can understand that you will be sending email using that person's account. I would be surprised if you could receive that person's email without their password.
Email applications can use smtp auth to enforce the use of a password when sending email. Go to http://www.qmail.org/top.html
and do a search for smtp auth. You will see some options.
I would recommend setting up a test server and getting smtp auth to work on it first. You should only modify your production server when you have everything working properly.