(Translated by https://www.hiragana.jp/)
Apple - Mail - The smartest email app you’ll ever use.
The Wayback Machine - https://web.archive.org/web/20110623030046/http://www.apple.com/macosx/whats-new/mail.html

Mail

First-class email.

Get ready to change the way you interact with your email every day. OS X Lion takes a few cues from Mail on iPad, then taps into the power of the Mac to deliver the best email experience you’ve ever had on a computer.

See more of your messages.

Now you can really take in your inbox. Mail displays your messages and open email side by side in a new widescreen, two-column view. The new favorites bar below the toolbar works much like a bookmarks bar in a web browser. It gives you one-click access to your mail folders, displaying them and their unread counts without cluttering up your Mail window. To add folders, just drag them from the sidebar into the favorites bar.

The power of preview.

A compact preview of each message — including the first two lines — lets you scan your messages at a glance. You can customize your previews to show more lines of each message, and even opt to show photos of the people who sent them (imported automatically from Address Book).

Search smarter.

Tame your inbox with powerful new search features. Search suggestions help you find messages even if you’re not sure what you’re looking for. Just start typing whatever you remember — a recipient’s name, part of a subject line, a keyword — and Mail dynamically suggests matches. Search tokens let you refine what you find. Say you do a search for John Appleseed. Mail displays a token in the search field. With one click of the token, you can instantly narrow your search results to include just the messages to or from John. Or only the ones where he’s mentioned. You can combine multiple tokens to find the tiniest needle in the biggest haystack.

Keep the conversation flowing.

Mail introduces Conversations: a new way to read and manage email that automatically groups messages from the same conversation. Fully compatible with other email applications, Conversations reveals streamlined feeds of messages in chronological order, so you can see how each thread has progressed without having to search for individual messages. Redundant text from previous messages is hidden so the thread is easier to follow, and graphics and attachments are retained as they were originally sent. And you can easily file or delete an entire conversation. So you’re managing a chain of ideas, instead of a few scattered thoughts.