A small Silver Spring computer company has signed a major agreement with the British Post Office to develop and market an electronic mail system throughout the world, it was announced yesterday.

The company, Dialcom, Inc., was chosen after the British quasi-government postal organization investigated the mail systems of more than a dozen North American companies, including subsidiaries of much larger firms, such as General Telephone & Electronics Co. and Hewlett Packard.

The agreement represents not only a significant step forward for Dialcom, which expects 1981 sales of about $8 million, but also for the British postal and telecommunications business. Development of the electronic mail effort is considered a vital step in the British government's effort to build a competitive telecommunications industry.

"This is a package that has global possibilities," said Robert Ryan, president of Dialcom and one of the founders of the 11-year-old company. Ryan said the agreement is the first of its kind between a major postal system and a computer systems concern in which the two organizations will share the project's revenues.

British Telecom, the telecommunications arm of the British Post Office, will market the system throughout the United Kingdom. In addition, Dialcom and the British organization will jointly market the service to telephone and postal concerns in other European nations and around the world.

Although the British are considered substantially ahead of American development of text systems sent to videoscreens--or videotext and teletext--this country's communications industry is well ahead of the British in electronic mail systems. Ryan suggested that the gap may exist because the United Kingdom is thought to have a more efficient, conventional mail system than does the United States.

The company will receive a fee for the computer software used in the program and will share the revenues. Neither organization disclosed further financial details.

In essence, electronic mail systems permit users to electronically receive and send messages over computer networks. The British system will also link terminals with data bases that allow customers to display airline schedules and news stories, for example, on their terminals.

In this country, the Dialcom system has about 30,000 users in 1,100 companies. The company claims to serve about 30 percent of the electronic mail market, with a client list that includes Westinghouse and Eastman Kodak. Westinghouse, for example, has about 1,200 "mail boxes" or users of the service. The company also operates a pilot program in the U.S. House of Representatives.

Although Ryan said in an interview from London that the British contract is unlikely to quickly generate revenues to match major corporate clients, it is the first step in what he said will be a series of contracts with other government or quasi-government postal concerns.

He predicted the privately held company's growth over the next two years would be "extremely rapid" and that the potential for a global mail system could lead Dialcom to continuing, steady growth over the next decade.

The company was formed in 1970 to serve basic business computer needs, such as providing payroll, ledger and engineering applications. The company's business began to shift in the mid-1970s, Ryan explained, into message services and, ultimately, into operations that have come to be called electronic mail.

Ryan said that among the reasons his company was chosen for the British pact is that the Dialcom system can hook up with a broad range of equipment, from telex machines to complex computer terminals.