Mail.app is the default e-mail
application for Mac OS X
It's similiar to the Mail.app that was included in the NeXTStep OS, and is an early port from that OS. It's written in the Objective-C language, which the Cocoa API in OS X. That supposedly allows it to run faster as well as lets it use Mac OS "Services" like Systemwide Spell-checking and PGP / GPG. Many of those features were present in the older NeXT version. Although its now in OS X's Aqua theme, in earlier developer previews, users could hack the resource files to force Mail.app to revert to its original NeXTStep skin.
It's been present ever since the first public beta of the Operating System. It's icon is a blue postage stamp with a photo of a bird. It's stamped with a cancellation mark, which reads "Hello from Cupertino" and an Apple logo. That's where Apple's headquarters are located.
Mail.app is Apple's replacement for Microsoft's Outlook Express, which wasn't ported to OS X. It's a very popular client for OS X, since it's free and bundled with the OS. Popular alternatives are Microsoft's Outlook for OS X, titled Entourage, as well as Eudora or Mozilla.
Although present in OS X since the beginning (as far back as Apple's private Developer previews), it's gone through several revisions:
Mac OS X Public Beta
The Mail.app had the ability to read HTML mail and had a sliding drawer on the side to organize the various mail folders. The application icon said 1.0 on it. Also, the toolbar on the top of the app had giant square Aqua buttons. It made it look really easy to use, but some people hated it. It was a public beta, and they got lots of feedback on Mail. It already could handle iTools' Mac.com email addresses (now known as .Mac. It also had the ability to "bounce" email back to a sender, which would generate an error message on the other side. It was a great anti-Spam tool, since you'd typically be taken off the email list.
Mac OS X 10.0
This was Apple's official 1.0 release. A lot of their feedback went into effect. As the Finder shed the huge icons, so did Mail. The "Customize Toolbar" menu replaced it, and you could drag and drop to reorder your toolbar, with icons, text, or both.
It became integrated with the OS X address book. Also, the number of unread messages appeared in the Dock icon in a red circle.
Mac OS X 10.1
Released 6 months after the 1.0 release, it had lots of bug fixes and people wrote plugins to sync it with a PalmOS device. In addition, Mail got better filters so you could file or color-code messages according to rules you set up.
Mac OS X 10.2 Jaguar
Mail.app got some great upgrades, though its version number was only 1.2. High up on the list was the new Junk mail filter that uses something called semantic filtering to figure out which messages are Spam and what aren't. You the user calibrate it in the beginning, using "Training mode" and then set it to put suspected Junk mail in the "Junk" folder. Most users say it works splendidly, and I have no complaints and extremely few false negatives. It is also integrated with Apple's Address Book and iChat, so you can see names next to messages as well as see if the sender is online. In addition, it can now display Quicktime movies embedded in the email.
Mac OS X 10.3 Panther
"We've really improved Mail for Panther" said Steve Jobs when he previewed it at the WWDC 2003. Some new features are HTML rendering using the Safari browser's API (which takes a lot from the Konquerer browser), Addresses are "objects" that can be drag and dropped and have contextual menus, and E-Mails can be organized and viewed in "threaded" form, and improved Junk Mail filtering that can also work with your ISP's junk controls, as well as junk mail privacy that prevents spammers from knowing you received their mail. You can also choose to permanently add a BCC field for newly composed emails, or empty your Junk Mail directories with just one click. Mail has support for Exchange accounts, as well as the ability to define an Outlook Web Access Server. You also have the ability to encrypt email via the S/MIME security standard as well as sign and validate the authenticity of email addresses with an S/MIME certificate. It's also "Kerberized".
Mac OS X 10.4 Tiger
Mail, despite the signifigant upgrades, is now labeled Mail 2. At WWDC 2004, Steve Jobs showed off some new features of Mail's latest iteration. It integrates with the new Spotlight feature built-into the OS, allowing for searches from the OS and in the app itself. In the demos, it manages to search through tens of thousands of messages in less than 2 seconds. Tiger Mail has supposedly switched to an “exploded” Maildir-like storage format with a single message per file. However, the Mail.app version in OS X 10.3 also did this for IMAP mailboxes only (it used mbox format for POP accounts). New "Smart Mailboxes" copy the "Smart Playlists" feature in iTunes, allowing special mailboxes that filter messages based on criteria you give it and are updated on the fly. The definition is complicated, but you can now create folders that only contain letters from a certain email address or mentioning a specific topic, for example. The mailboxes display messages based on easily-customizable mail rules. Apple adds "Unread" and "Viewed Today" mailboxes by default. All smart mailboxes work over multiple mailbox accounts, making them all the more useful for users who must handle multiple e-mail accounts and inboxes. The drawer is different as well, now users have the option of a malbox bar, similar to Safari's bookmarks toolbar, where a user can click on a mailbox or Smart mailbox. It also has account setup assistants and priority flagging of email. Mail signatures is updated, and the Mail windows allow more advanced fields, like Reply-To and BCC, which were harder to get at in earlier releases. It also tries to simplify tasks like sending images in email, offering to resize images on the fly, and offering to present received images into a slideshow, or adding to iPhoto with just one click and a nifty animation. Other features included changing recipient addresses to red as a warning when sending email outside the user's domain (useful for jobs where you can't discuss business to non-employees).
Mac OS X 10.5 Leopard
Announced at WWDC 2006, Steve Jobs told the audience how he is a heavy user of Mail.app (likely since its NeXTStep days), and uses it for far more than messages. You can now create Notes to hold information and synchronize them with your iPhone. Similar to post-its or Apple's Stickies app, they feature a yellow background and can hold images and text, as well as checkboxes. All Notes are put in a special Notes mailbox, and any text you highlight in a note or email message can be turned into a To-Do item; letting you give it a date, priority, or iCal calendar. Mail.app now ties into iCal, letting you email To-Do items to others, and ones created in Mail now appear in iCal. A To-Do created from an email is hyperlinked to the initial email, allowing you to keep track of messages better. It also has new data detectors (like GMail) where it can parse out an email and create a calendar event based on the date and time in the message, or a new contact based on a signature. Mail also has a new Stationary feature with 30 customizable HTML email templates, letting you drag-and-drop images or movies. Apple also added an RSS reader into Mail, letting you filter messages the same way you filter E-mail and giving you an RSS mailbox. The notes and mailboxes also sync across multiple Macs with the .Mac service. Apple also announced anti-Phishing security features, but won't show them until Leopard is released October 2007.
AppleInsider.com has a timeline of Mail iterations: http://www.appleinsider.com/articles/07/10/15/road_mac_os_x_leopard_mail_3_0.html&page=1