DON'T let anyone tell you that two-way wireless messaging is a new technology -- using smoke signals, American Indians once sent instant messages to colleagues miles away. Of course, smoke signals had their drawbacks: reception was spotty in rainy weather, Microsoft Word attachments almost never came through, and the blankets got all sooty.

Thanks to a recent invention that may best be described as an e-mail pager, the situation has improved enormously. These devices send and receive e-mail wirelessly, indoors or out, from almost anywhere in the country.

Unlike previous generations of two-way pagers, the new ones have built-in keyboards. True, the keys look as though they have gone through the wash in someone's jeans. But even in its shrunken form, the keyboard works surprisingly well.

By gripping the pager with two hands and flailing away at the keys with both thumbs, most people average about 15 words per minute. That's about the speed you get on a Palm hand-held if you're good at its Graffiti alphabet -- but the keyboard never misinterprets the letter you want. As a bonus, you'll soon be able to take on any thumb-wrestler in the joint.

Even so, the non-gadgeteer may wonder what the fuss is about. Haven't the thumb-typers ever heard of cell phones?

But e-mail pagers offer some substantial advantages: they're never busy, they never crackle, and they use a fraction of the battery power. Because you can remain silent and don't have to press the pager against your head, it is less obnoxious to use in public. (It's perfectly possible to read or write e-mail in a meeting or in your opera seat, completely undetected.)

Furthermore, when you travel out of range, incoming messages wait on the network. They pile into your pager automatically when you return to civilization. And the service is far less expensive than cell-phone service, costing as little as $10 per month. (Of course, cell phones have advantages, too. For example, you don't need both hands. Oh, and you can talk.)

The low end of the product spectrum is filled by Motorola's Talkabout T900, which comes in black, aqua, dark blue or red plastic. When closed, it looks like an ordinary beeper. When you open the lid, however, it looks like an I.B.M. ThinkPad for hamsters: tiny screen on the top half, keyboard on the bottom.

This pager, like all of those reviewed here, can either vibrate or play a melody when mail arrives. It has its own e-mail address (like, but a $30 Windows program called MyMail Desktop, new this week, can automatically forward mail from your existing Microsoft Outlook account to the pager. (Motorola says that an update later this year will accommodate other e-mail programs.) You read, forward, reply to or delete the messages using simple on-screen menus.

A sizable percentage of the T900's one million users are under age 20, thanks mostly to the pager's low price. In theory, the device costs $200, but deep discounts are almost always available. For example, Motorola's Web page offers it for $100 (after a rebate) through June 25. You also need to sign up for network service, just as with a cell phone; Verizon, for example, offers various plans from $10 per month (for 125 messages) to $20 (400 messages).

It's easy to see where the corners have been cut, however. The screen shows only four lines of text at a time. You have to press a Shift key to type punctuation and numbers. You can't send messages longer than about a paragraph, and incoming messages are chopped off at that length. And the T900 doesn't synchronize data with a PC. Its address book holds 250 names, but you have to type them in by hand (or by thumb).

To escape those limitations, you need the T900's big brother, Motorola's Timeport P935 ($400). Its grayscale screen offers icon buttons and a choice of fonts. It's bigger, too, closer to a family-size bar of soap than a hotel-sink size. The extra bulk accommodates a more complete keyboard with number keys. Like the T900, the P935 doubles as an ordinary pager: e-mail-challenged colleagues can dial your 800 number and punch in their own phone numbers for callbacks or can dictate messages to a friendly, accurate operator, who sends the transcribed messages directly to your pocket.

The P935 runs add-on programs, plays games, sends faxes (for an extra fee), offers the usual organizer amenities (calendar, to-do list and so on) and, via its cradle-recharger, synchronizes your e-mail and addresses with Microsoft Outlook or similar programs. According to Motorola, its MyMail Desktop Plus software ($50), due this summer, will let a P935 (or T900) issue wireless commands to your Windows PC, telling it, for example, to send a certain file as an attachment to a colleague -- a genuinely useful stunt for the absent-minded traveler.

Perhaps the autotext feature is the one you'll use most frequently, however, because it expands abbreviations into much longer strings of words. For example, you can set up an entry so that typing ''CTRN'' spits out ''Can't talk right now. I'm typing on a computer the size of a brownie.''

All of those extras, unfortunately, saddle the P935 with considerable complexity. Its brain seems to struggle with some tasks, hesitating several seconds before the screen changes. While the T900 succeeds because it's so focused on its mission of being fast and cheap, the P935 wants to be a Palm organizer but lacks the power and software simplicity.

No such problem afflicts the RIM BlackBerry, whose thoughtful design and lightning-fast software has made it a million-seller in the corporate world. (You can't ride the Metro-North commuter train into New York without overhearing some BlackBerry user explain to a seatmate: ''It's better than a Palm VII. You don't have to check for e-mail -- the mail comes to you!'')

The BlackBerry comes in two sizes. The smaller (Model 850 or 950) resembles a typical belt-clip pager. The larger (Model 857 or 957), with its much bigger screen, could easily be mistaken for a Palm. The crisp black-and-white screens are not touch-sensitive, however. You navigate through on-screen options by using a thumb wheel.

The software is brilliantly engineered to save you thumbstrokes. For example, pressing a key for more than one second capitalizes the letter. Similarly, you can press the space bar twice to end a sentence, which gives you a period, a space and automatic capitalization of the next word.

It's important to note that the corporate (''Enterprise'') and personal (''Internet'') editions of the device work slightly differently. For example, the corporate version's desktop software (called BlackBerry Redirector) synchronizes your pager with your Outlook or Lotus Notes mail wirelessly, much like Motorola's MyMail. A server-software upgrade released last month also permits live, wireless calendar syncing. If your secretary changes an appointment on your desktop PC, the change appears automatically in your BlackBerry's datebook program.

The Internet edition, on the other hand, gives your pager its own e-mail address. Without jumping through some technical hoops, you can't get mail from your existing e-mail account unless your Internet provider is one of a few, like EarthLink or America Online, that sell the BlackBerry preconfigured for their own e-mail services. (AOL's recently released model, called the AOL Mobile Communicator, costs $330. For $20 per month, it offers not only AOL e-mail access, but even wireless instant messaging, a sure boredom-killer for the mobile teenager.)

Riding back home on Metro-North, you'll hear only a few complaints about the BlackBerry. The backlight illuminates only the screen, not the keys, making it tough to do e-mail at the Met. The built-in information-management programs are no match for, say, the Palm's. The address book screen, for example, lists your contacts' names but not their phone numbers.

Finally, there's the matter of price: $400 for the small BlackBerry, $500 for the bigger one, and a mouth-drying $40 per month for the e-mail service (make that $60 monthly if you want Web access, too). Add another $6 monthly for paging service. Those are unlimited, all-you-can-surf prices, but daunting nonetheless.

With any of these pagers, expect a few glitches. Service is patchy in rural areas, so when you sign up, ask about reception where you live. And don't expect instant communications -- the messages you send sometimes take several minutes to arrive.

Otherwise, these products nail a niche somewhere between beepers and cell phones. For certain people, e-mail pagers may grant a surprisingly liberating gift of convenience, savings and soot-free bedding.

Photo: FOR NIMBLE THUMBS -- The thumbs are used to send and retrieve wireless e-mail on the BlackBerry 957, left, and the Motorola Timeport P935. (pg. G8) Drawing (Stuart Goldenberg)(pg. G1)