Allow an user to send as any other user.

==== Required information ====
- iRedMail version: 1.8.2
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Linux/BSD distribution name and version: Debian 7
====

First of all i'm sorry for my english.
I've one question about how i can authorize an user to send as any user in server.

I've a web app that send an email using the loged user email, but the server don't allow it because it uses one account. I've changed the main.cf from:

smtpd_sender_restrictions = permit_mynetworks, reject_sender_login_mismatch, permit_sasl_authenticated

to:

smtpd_sender_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_sender_login_mismatch

and works fine, but i want to allow only that user to do it, not to everybody (even spammers). Is there any way to do it?

Thanks!!