You can install only Antivirus and Antispyware component on this server, Do not include, Email Plugin..
that is for users only.
I found this on the site,
Q Which software would you recommend to protect our users from viruses?
A PLEASE, do not use normal, memory resident antiviruses on the computer where you are running mail server. They do not protect your users. They try to protect you, but creating problems instead, because they are locking files and portions of memory, when they find viruses in email messages, and may crash mail server.
For Pro version, you can use one of 3rd party server extensions (plugins). Check them out here..
Also, since it acts as a gateway between your server and the Internet (your server must be running on non-default SMTP port), you will be unable to use e.g. SMTP Authentication, to protect your server, also, if you are planning to use blocked and/or trusted IP addresses, and other security features of our server, it will not work, since our server will not see original IP addresses of connecting computers.