Postal employees in Utah and throughout the United States were to complete the biggest mailing in Postal Service history Thursday, when 116 million copies of "Your Guide To Complete Addressing" will be delivered to every address in the nation.

Utah postal officials say the four-page guide is designed to increase customer cooperation in addressing mail. They say such cooperation is needed because the Postal Service is changing the way it processes mail.About 52 percent of all letter mail is now sorted on automated scanning equipment, and by 1995 virtually all of that kind of mail will be sorted with automatic equipment.

Salt Lake City Postmaster Kenneth R. Prentiss said the address on a letter or any piece of mail, whether it needs a rural route or an apartment number to be complete, is still the best tool the customer can give the Postal Service.

"Help from customers who use a complete, correct address, including ZIP code, lets us take full advantage of our high-speed, electronic mail-processing equipment."

He said the use of clear, complete and accurate addresses will cut delays that occur when a post office must make a correction. "And it will keep processing costs and postage rates lower for longer periods of time," he said.

Locally, the Salt Lake City Post Office uses four optical scanners to read addresses and apply bar codes, and five other machines that read bar codes that have already been applied.

Nationally, there are more than 400 advanced scanners in use; another 346 will go into use over the next two years; and there will be another 8,000 pieces of automated sorting equipment in use by 1995, Prentiss said.

Postal officials are urging customers to find out the four numbers that follow their regular ZIP code, such as 6789 in the nine-number ZIP code 12345-6789, and use it on their stationery, business cards and checks. This nine-number ZIP code on mail helps pinpoint the side of the street or the apartment or, in some cases, the suite in a business building, where an addressee lives or works.

Also, postal officials urge patrons to use correct abbreviations in addresses, such as AVE for avenue, RM for room, ST for street and so forth so the electronic machines can more easily sort letters.

The guide being delivered this week also lists standard abbreviations for states that are different from those familiar to many people. For instance, the Postal Service machines are programmed to read AL, not Ala., as Alabama (and no period after L), and UT for Utah, instead of the name spelled out. FL, not Fla., is Florida, and PA, not Penn., is Pennsylvania.

And, if an attention line is needed, put it on top of the address to which you want your letter sent, not at the bottom lefthand corner of the letter, as was once the custom.

The electronic machines the Postal Service use scan the bottom of each address for cities, states and ZIP codes, and an attention line at the bottom of the address or at the bottom of the letter will only delay the piece of mail.