PRESIDENT REAGAN called the home of an 84-year-old Los Angeles man the other day to offer apologies for overzealous fund-raising by some of the organizations that had been working to keep the president and like-minded people in office. The man, whose name is Gerald Colf, responded to the mailings of 27 conservative groups by sending in some $4,200 -- all of his savings -- before his daughter learned about it and began efforts to get the organizations to return the money to him. So far about $1,500 has been recovered.

Perhaps it was an unavoidable misunderstanding. Mr. Colf, having been born around the turn of the century, knew a time when letters were an important means of communication between human beings who actually wanted to convey information to one another, and he may well have developed the habit of taking his mail seriously -- to the point of opening and reading all of it. That was once considered an elementary courtesy; now it is considered a waste of time.

Now there are computer-generated mass mailings with the recipient's name strategically inserted in six different places in the letter. There are cross-pollinating mailing lists and a thousand interested parties -- conservationists, right-wingers, left-wingers, evangelists, demagogues, worthy charities, unworthy charities, tub-thumpers, fanatics with a vision of the New Jerusalem -- who have computers capable of probing for any shred of interest you might ever have shown in their cause and sending you a letter to ask for money.

The letter might be personal, touching, beautifully written -- and contain a personal snapshot of someone with his favorite beagle, both of whom would like your contribution so they can continue their fine work for the republic. The first line of defense against this as people become ever less inclined to take their mail seriously is to throw the stuff away unopened. The mass mailers are counterattacking by trying to make their product look like anything but what it is -- like a check, a summons, a letter from Mom, maybe someday soon a ransom note -- in increasingly futile attempts to get people to open the envelope.

The president didn't reach Mr. Colf, who recently moved to a retirement home, but he talked to the man's daughter, Judy Kerrigan. She said Mr. Reagan offered his apologies and told her that if they got any more such mail they should throw it in the trash. Good advice for all of us -- should've been in the State of the Union.