I’ve been using Mail.app for about 3-4 years now (ever since I got my first Mac) and something that makes me continue to use it is how well it filters out the junk mail. In July of 2006, BBC reported that 95% of email is junk mail. That’s a lot! I would say on an average day I get over 250 spam emails. Mail manages to filter out 99% of them so only 4-5 get them into my Inbox. That’s not to bad though!
Mail comes with built-in junk mail prevention. When you first start using mail it will be in a training mode. This mode is really on all the time even when you set it to Automatic mode. The difference is that training mode will just mark your mail as junk and won’t move it into the junk mail folder. This is good because if something is marked as junk that isn’t junk you can un-mark it and mail will learn from that mistake. Of course in automatic mode, you might want to sift through the junk mail folder on occasion just in-case it marked some mail as junk that shouldn’t be, it will learn from that too. At some point which I cannot seem to find out a dialog box comes up and prompts you to switch into Automatic mode or you can leave it in training mode.
I am quite impressed with how Mail handle spam. I use the mail server that comes built in with OS X Server for my email also and it too has spam prevention. Having that working in conjunction with Mail.app really saves me a lot of time (now I can read emails and not spend time sorting through hundreds of spam and non-spam emails).
There are other things you can do about getting rid of spam from your inbox. A program called Spam Sieve works with your existing mail client to get rid of spam. There’s a nice article here about Spam Sieve. You can also check with ISP to learn about how they prevent spam from reaching your inbox.