Note: with the possible exception of the sendmail milter libraries, most current-day versions of Linux come with all of these required packages either pre-installed or readily available through their package manager. You might wish to explore that avenue before downloading and compiling the packages from scratch.
On the other hand. If you wish to run the absolute latest version of some of this software (e.g. SpamAssassin and sendmail), you may wish to persue the source/compile option, since many of the OS-distributed packages lag behind by several revisions.
No filtering of mail messages can be expected to come for free, especially not the extensive level of filtering that is done by MailCorral. However, the filter was built with performance in mind and it has been shown to perform very well in production environments.
A typical user of MailCorral is an ISP, with thousands of users, who runs sendmail on a dedicated (or mostly dedicated) mail server. In real environments, we have observed sustained loads of 2-4 email messages per second (which translates approximately to 10,000 messages per hour or a quarter million messages per day). The performance numbers stated below are typical for server loads such as this and you can expect similar numbers on your server.
Adding filtering to the email server increases the load on the system such that the filter (plus the spam and virus arbitrons) consumes between 25-45% of the CPU. The filter itself usually accounts for 15-20% of the CPU and the typical spam arbitron (e.g. SpamAssassin) usually accounts for 5-10% of the CPU. The virus arbitron can account for another 10-15% of the CPU.
To put it another way, if your CPU is more than 55-60% busy running sendmail, you may experience some performance problems by adding filtering. If it is less than 50% busy, you should see no impact. When considering an upgrade to a machine running flat out, a 65% increase in horsepower should easily accomodate filtering.
The guidelines above should help you in planning your mail filtering setup. If you would like more specific performance numbers, please contact BSM Development and tell us what you'd like to see.