Postfix-how to restrict outgoing mails to Local eccept one domain

[sohan]

Hi all,

I am new to Linux. My setup is like this : Installed LAMP, postfix,devcot...all running properly. My server is also serving as Firewall(iptables & Transparent Squid)

My problem is I want to restrict outgoing mails to LAN only. Our domain is hosted at ixwebhosting. I named my Linux domain as the same name (xxxxxsoftware.com)

Is it possible to allow outgoing mails to only xxxxsoftware.com and deny to other domains ?

I have created couple of local users who access mails from my linux box. I don't want them to send mails outside world but allow only xxxxsoftware.com...

my mail.cf is :

# Debian specific: Specifying a file name will cause the first
# line of that file to be used as the name. The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname = xxxxsoftware.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mailbox_command = procmail -a $EXTENSION
mailbox_size_limit = 0
recipient_delimiter = +
mydomain = xxxxsoftware.com
relayhost =

please help me....

Thank you
SOHAN