As for some Mid and small company, Synology that integrate DSM allows to install/implement a mail server and others… 🙂
The Mail server is based on Postfix, one of the most common Mail server as an alternative to sendmail.(http://www.postfix.org/).
As spam is everywhere, you might consider to implement some solution as Brightmail instead of the spam assassin which is indeed also part of the available package from DSM.
To do so, add these lines to your main.cf from Postfix:
smtpd_milters = inet:IP:41001 milter_protocol = 2 milter_default_action = accept
IP: is the IP of the machine that have Brightmail software.
On Synology, the file is located to:
You might also wish to implement the change into the template file, as if some postfix update are applied this settings might be overwritten.