I have a assignment of configuring outgoing email filtering on ubuntu 14.04. We have incoming email filtering is enable using postfix and spamassassin. I have searched google but does not find exact answer while some searches gives some clues to some extent of solution. What I need is please give me a complete layout of what should I have to be done in whole.
Thanks
I believe this has already been answered on ServerFault https://serverfault.com/questions/126282/postfix-check-outgoing-mail-for-spam.
You can use spamassassin for outgoing as well as incoming filtering. It can be used also for internal mail. While it is commonly disable in both these cases, it is useful to know how to enable this functionality in case of spam outbreaks on your systems.
Other approaches used include throttling send rates, content and virus filtering, and authentication. On Linux (Ubuntu) you can also limit the ability to send traffic on port 25 (SMTP) to postfix in the firewall.
Your firewall should allow only your authorized mail servers to send email to the internet. These should have appropriate DNS setting such as rDNS and SPF records. Your organization may also want to have DKIM and DMARC records for the mail domain.