I'm going to be setting up an email server for my company, which is rather small. It will only have about 15 email accounts. Currently the email provider we are using has a terrible spam filter and many of the people receive in excess of 200 spam a day. So I'm just wondering what is the best solution I can do to solve this. I tried qmail (from the qmailrocks.org tutorial), and it worked fantastic for a little while, but eventually it ran into a problem where mail wasn't delivering (stuck in the queue) and I could not fix it. So now I'm thinking just use sendmail + spamassassin. Are there any addons for this where you can manage domains/users through web browser? Is there another email service that I should use?

This is not necessarily limited to free software, I have some funds at my disposal if need be. The OS will probably be Slackware 10.2, but possibly FreeBSD. Thanks in advance for any advice!

whilst sending is the most common and most mature SMTP server, it's horribly complicated to configure. When you so configure it don't even edit the configuration file itself. you actually edit a metadata file which is then processed into the configuration file becuase only 2.5 people in the world can understand the file. yuck. Personally i've had good success with both postfix and exim, but i'd choose postfix of the two. this can be integrated with SA in a number of ways, to suit your needs. One big plus for exim though was that things like sorting into folders and other weird things are done within a single daemon and config, whereas other mta's would need to shell out to procmail and other tools.