SAN JOSE, Calif. -- U.S. business loves a gadget, and it has found a great one in electronic mail.

With millions of computers now on office desktops, "E-mail" -- messages and other computer files sent from one machine to another -- has become a familiar part of the business environment. Like the telephone, it's both depended on and taken for granted.

E-mail, again like the telephone, creates its share of annoying problems. Nonetheless, it's quickly evolving into a tool that is vastly more complex than its original use as a high-tech version of the Post-It note.

Companies are using E-mail to handle substantive business matters: communicating key corporate decisions or routing paperless versions of corporate forms such as invoices and purchase orders. And firms are giving customers and suppliers access to their corporate computer networks so they can stay in touch on E-mail.

Companies with international operations say E-mail is the only good way to keep up with far-flung employees, especially since letters, spreadsheets and other files can be "attached" to E-mail messages. And many business cards these days sport something new: the owner's E-mail addresses.

"In the electronics industry, if you're not on E-mail, it's almost like you're not a member of the community," said David Fickes, who works in the communications department at Sparc International, a Menlo Park, Calif., workstation consortium.

Mr. Fickes uses E-mail to solicit articles for his newsletter and to pass along news tidbits to colleagues at workstation companies.

Another enthusiastic E-mailer is Businessland Inc.'s Chuck Stegman, who uses it to answer questions from customers. Since he often reviews his old correspondence, Mr. Stegman has stored on his hard disk his E-mail messages from 1990.

All 15,000 of them.

That turns out to be 60 messages a workday, only slightly on the high side, since 30, 40 and 50 messages a day are quite common.

While some firms prohibit non-business uses of E-mail, others actually encourage it, often giving employee social groups their own electronic mailboxes.

At the San Jose Mercury News, for example, reporters can send an E-mail note to all other reporters. Typical traffic ranges from requests for help with a story to fulminations against the latest outrage by this or that editor.

E-mail boosters say the informality of the technology encourages people to pass along ideas and bits of information that might not, by themselves, merit an appointment, phone call or memo. And they say it leads to fluid corporate structures because companies can use E-mail to set up ad hoc groups.

Not all firms have embraced E-mail, though. Some don't have their computers networked together. Others find current software too difficult to use.

And Nina Burns, president of Network Marketing Solutions of Menlo Park, Calif., said many old-school firms resist E-mail because workers use it to interact without regard to corporate hierarchies.

Such firms are becoming the minority. According to International Data Corp., about 3.5 million desktops are hooked up with electronic mail, and sales of E-mail software products should quadruple, to $350 million, by 1994.

E-mail users include most of the well-known names in Silicon Valley, though they use it in disparate, culturally revealing ways.

At the somewhat traditional Hewlett-Packard Co., for example, President John A. Young prefers to have his secretary bring in a printout of his E-mail messages, which he answers through dictation. His secretary then transcribes the responses back into E-mail.

But the heads of younger and more freewheeling companies tend to be more hands-on with their E-mail. Both Apple Computer Inc. Chairman John Sculley and Sun Microsystems Inc. President Scott McNealy grind away at their E-mail themselves.

As does Harvey Jones, president of Synopsys Inc., a Mountain View, Calif., semiconductor design software firm, who is the very model of the E-mail manager.

"When we first got E-mail, a typical message might have said, 'I'm about to finish a proposal. Can we have a meeting to discuss it?' Now a message might say, 'Here's the proposal. Can you review it?' E-mail has become a major means of communicating on very substantive issues."

"Neither E-mail nor voice mail substitutes for face-to-face meetings on important subjects," Mr. Jones said. "But with it, I make a lot fewer intracompany phone calls than I did a year ago."

E-mail is old hat to academic researchers -- whose computers have long been linked by a Defense Department-funded network -- and to employees of companies using the Unix operating system or certain IBM mainframes.

What's changed in recent years is that personal computers have been hooked to each other on networks, allowing E-mail to flourish in offices. And when modems are used, one computer user can send an E-mail message to just about any other one.

With personal computers comes a potential E-mail market of 50 million to 60 million machines, prompting industry powerhouses such as Microsoft Corp. and Novell Inc. to look for E-mail products to sell.

In some cases, they got a product by buying a company, as Lotus Development Corp. did in February with CC:Mail of Mountain View for an estimated $25 million.

Another sign of an E-mail boom is that specialized, second- and third-generation products are being developed. For example, Reach Software Corp., a Sunnyvale, Calif., start-up, is creating E-mail programs targeted exclusively at advanced business users, according to Chief Executive Officer Anand Jagannathan.

David Atlas, an International Data Corp. analyst, said firms like Microsoft have ambitious plans for E-mail, seeing it as the ultimate way of delivering -- and billing customers for -- their software.

"That's the real reason the big companies are interested in E-mail, even if they don't like to talk about it," he said. "Once you own a delivery service, you can deliver more than just mail."

If future versions of 1-2-3 might one day be shipped on E-mail, its uses today are far more pedestrian. One of the biggest criticisms of E-mail is that it's too easy to send a note, leading to a deluge of messages of dubious value.

But "almost all the problems that exist with E-mail also apply to paper," said Walter Ulrich, a consultant with Arthur D. Little. "If someone is going to be sending too many memos around, I'd rather they do it on E-mail."