I did this in my office, since some users also send via our server that make our internet connection getting very slow.

In my office there are some indonet user which the ISP reject to use it's SMTP server if the connection use other than indonet line. So I have to setup our own SMTP server using postfix.

Ok, this is how to setup postfix to allow only specified users:

1. add line into /etc/postfix/main.cf:

smtpd_sender_restrictions = check_sender_access hash:/etc/postfix/sender_access,reject

# this mean tell to postfix to get from sender_access file for validating email, and "reject" at the end is reject all for default.

2. Create file /etc/postfix/sender_access and add allowed email sender and add "OK" after it:

iip@mydomain.com OK
joe@kupluk.com OK
kanotix.com OK

#the last one will accept from *kanotix.com

3. Create lookup tables for postfix:

postmap /etc/postfix/sender_access

4. restart postfix



REFFERENCE

http://www.muine.org/~hoang/postfix.html
http://www.postfix.org