Linux Server-wide Anti-spam Solution

for a server-wide anti-spam solution I can implement on a Linux server. The mail queues are constantly getting backlogged with thousands of messages which bring the servers to a crawl. There are really two issues the solution needs to address.

1) Spam
2) Spam sent to other people that have the from address forged with our clients' email addresses.

We have spam assassin installed for individuals to use, but there's got to be some sort of solution that can clear out the vast majority of the junk before it even gets to the queue.

It probably goes without saying, but the solution needs to be open-source or have a very inexpensive license.