Linux has an abundance of fantastic email clients. I personally don't ask for much from an email client; when I used MS Windows I used Outlook Express until I discovered Thunderbird
(which I use to this day). Other people are not so easy to please when it comes to email, and there is likely a Linux client available for those users, too.
As I mentioned above, I use Thunderbird which is also available for MS Windows. This program is very similar to Outlook Express in that it is a good, simple client with enough features for most users. Thunderbird uses an extension philosophy similar to Firefox, so if there is a feature not included in Thunderbird, you can likely "bolt" it on with an extension. It provides access to POP3 and IMAP servers, an RSS reader, filters, and local spam filtering that you can "teach" to identify spam by marking unwanted messages as such.
If you're coming from MS Windows and like using MS Outlook, you'll want to check out Evolution. As far as I know, Evolution provides nearly all the features offered by Outlook and plenty more. I have personally found Evolution to be more than I need from an email client, but calendar, tasks, and collaboration are all handled in a manner similar to Outlook.
Kmail is the default email client provided by KDE. I would group it in the same class as Thunderbird and Outlook Express, but it also offeres integration with KDE's task and calendar applications, so you'll also find some features found in MS Outlook.
Maybe it's just me, but Sylpheed Claws seems to have been gaining a signifigant following lately, too. It is based on another project called Sylpheed which uses the GTK toolkit (GNOME). Users tout it's speed and configurability as primary features. I've used it some because it is popular among Vector Linux users (my second favorite Linux distro) and have found it to be good, but I didn't use any of the additional features that Thunderbird doesn't offer and being a beast of habit, I went back to Thunderbird.
Popular CLI Email Clients: Mutt, Pine