In Lion, Apple is making some improvements to Mail. Mac OS X's native email client is pretty decent overall, hitting all the basics such as support for multiple accounts, easy setup, the ability to organize and search messages, and support for a range of mail rules or filters. But Mail is missing a few modern touches.
One big issue is that the view format is pretty much limited to a dual-pane display that shows a small list-box of messages above a preview pane, plus a sidebar with accounts and folders -- an interface that email clients have used for nearly two decades. The hierarchical folder view in the sidebar can make locating specific folders difficult and limits the ability to manage messages from multiple accounts. Similarly, the small message list-box makes scrolling to locate messages challenging.
Another issue is that although Mail offers decent search capabilities, specifying multiple criteria (by a word in the subject line, the sender's domain name, and to whom it was sent, for example) isn't really possible. Even getting granular with a single phrase or name and a specific folder or account isn't particularly user-friendly.
Apple says it will improve the view options in Mail by allowing a wide-screen approach, where you can have columns for all your accounts/folders as well as messages and a full-size display for a selected message. This option has been around in other email apps (most notably Outlook) for quite some time. Although Apple hasn't been too specific about search capabilities, it has said search in Mail will be refined. The screenshots available imply that the toolbar will be improved to make overall navigation better.
While there isn't much you can do to improve the toolbar and search capabilities of the current version of Mail, you can use one of two plug-ins to make the interface more user-friendly. WideMail and Letterbox do essentially the same thing: Replace the message list with a column that displays a greater number of messages and offers a larger message view next to it.
Both are good options; the differences between the two are minimal. WideMail is donationware, whereas Letterbox is free. WideMail offers some formatting options for how the message column is displayed, such as types of dividers, date style, and alternating background colors in the list. Both support Snow Leopard and Leopard; Letterbox also offers a version for Tiger.
A couple of other notable Mail plug-ins offer features that aren't quite Lion-like (at least from what we know about Lion at this point) but deserve some mention as well.
RelatedMail (free) offers a menu option that displays any messages related to the one you're currently viewing. The relationship is determined based on features such as message thread or subject, date or sender. It is free -- and technically still in beta, though it works pretty solidly -- and available for both Snow Leopard and Leopard. It doesn't provide the advanced search features Apple is promising, but it is pretty helpful.
GrowlMail (free) works with the Growl add-on for Mac OS X and displays new message details, complete with a small pop-up identifying the sender and subject and providing a short message preview. The effect is similar to the new message preview option in Outlook. GrowlMail works with Snow Leopard and Leopard.
Mac App Store
The Mac App Store, available to Snow Leopard users since early this year, places a wide, easy-to-browse selection of Mac apps at a user's fingertips without requiring a Web search. Purchasing, installing, and updating apps have been simplified to the one-click approach of the iOS App Store.