An email client is built into Mac OS X, and it's not a bad one at all. But you shouldn't be satisfied with this default email program at least not until you've checked out the free alternatives. Here are my top picks of free Mac email clients.
Mozilla Thunderbird is a fully featured, secure and very functional email client. It lets you handle mail efficiently and with style, and Mozilla Thunderbird filters away junk mail too.
The Opera email client is a slick and flexible interface that will satisfy almost all your email needs, and Opera integrates RSS feeds in this experience as well. Some may find the message editor lacking a bit in power, and the absence of support for encrypted email is unfortunate.
Mulberry is a powerful and incredibly versatile email client available for all the popular platforms. If there is something Mulberry lacks, it is simplicity.