When the piles of undelivered mail began to get out of hand, Emilio Vivaldi, a 30-year-old postman in the northern city of Imperia, got careless and started throwing some of it in the garbage. Late in January, post office investigators found over 500 pounds of mail stored in his cellar because, Vivaldi claimed, his pouch was too heavy.

The 6,000 or so pieces of mail, some of which dated back as far as 1973, will now be delivered to area residents with an official post office note of apology. Finding an explanation for the poor mail service that has plagued Italy as a whole in recent years has, however, proved far more difficult.

Postal services throughout the West currently are under fire for deteriorating service, largely due to rising costs and out-of-date organizational techniques.

Still, the Italian system has been especially inefficient in recent years. Although post office officials are optimistic that a turnabout is just around the corner, failure to apply good management practices coupled with the political inability to pass postal reform or adequate strike legislation have left the Italian mails highly unreliable.

According to Ugo Monaco, a small, bearded engineer who since 1975 has been director general of the Ministry of Post and Communications, the situation today has improved since the fall of 1978, when a Rome daily mailed out 114 test letters and noted, after a month, that 14 had never arrived, and that the average delivery times for letters, special deliveries and registered letters in Rome were, respectively, eight, 4 1/2 and seven days.

Seated in his spacious, wood-paneled office in the new giant ministry complex in Rome's modern EUR section, where a king-size phone console offers him instant communication with every Italian telecommunications center, Monaco proudly pointed out that recent attempts to better Italian mail service include the opening of 12 new centers of postal mechanization -- 11 more are planned in coming years. Italy has also opened 370 new branch post offices, for a total of 14,500, and new housing facilities have been built in northern Italian cities for the southerners who make up the bulk of postal workers in this country.

Other recent innovations, he said, were a reduction of postal grades from 24 to eight, a simplification of the bureaucracy involved in civil service exams, and a new labor contract involving productivity pay incentives designed to reduce persistent and massive absenteeism. Until recently absenteeism in some areas reached 25 percent on an average day.

The average now has declined to 9 percent in Italy as a whole and 15 percent in some areas of the understaffed Italian north.

Despite these small signs of recent improvement, however, horror stories still prevail. A book airmailed from Washington on Dec. 28 arrived here only after 32 days. One resident foreigner recently received several packages sent from New York last June. And a check mailed from Rome last November has yet to arrive at its destination in Milan.

One day last month, a major post office in Turin called in a private firm to help reduce a backlog of 40,000 packages.The elimination of piecework incentives in the Rome telegraph offices last month led to a slowdown of several days that delayed most telegrams for between 48 and 74 hours.

Since the economic boom of the 1960s, chronic disservice by the Italian postal system -- which moves 10 million letters and postcards daily -- has wrought havoc on Italy's burgeoning mail-order business and has forced industry here to spend hundreds of thousands of dollars on sophisticated communications equipment.

More generally, photographs of stacked-up, unsorted mail are common. Telex facilities are booming, telephone lines between major cities consistently are overcrowded, and the country's 87 private mail delivery services -- used by businesses, foreign embassies and government ministries -- do a flourishing and profitable trade.

The need for rapid and efficient communication also has led people here to rely on illegal out-of-town mail service provided by couriers who, by Italian law, are allowed only to handle packages. Friends and relatives frequently are pressed into service for hand-carrying missives to other European capitals. Many Rome residents writing abroad brave mid-town traffic to take their mail to the Vatican post office in St. Peter's Square, where rapid sorting is thought to save a day or two of otherwise wasted time.

Post and Telecommunications Minister Vittorino Colombo once startled an American journalist when he said mail service was difficult here "because Italy is a long country," and easier in the United States, which is geographically broad.

The reasons behind Italian postal chaos of the last decade appear to be far more complex, however.

"We have been waiting since 1948 for a postal reform bill to be passed," said Guiseppe Mastracchi of the powerful, left-wing General Confederation of Labor. He complained that basic postal structures here are the same today as they were when Italy was still a predominantly agricultural society.

Monaco agrees that the overall reorganization Italy's politicians keep putting off is necessary, and also concedes that mechanization ought to have been begun many years ago. But he blames much of the recent chaos on the excessive strikes in Italy in recent years. "Air strikes, transport strikes, and general strikes affect the mails as much as postal strikes do," he said.

Another persistent handicap is a lack of adequate personnel. Only about 80,000 of the ministry's 200,000 employes are assigned to the postal sector. Post office officials say 20,000 more people must be hired if service is to be improved and if a second daily delivery, eliminated a decade ago, is to be restored.

According to Alfredo Solustri of Confidustria, the Italian national manufacturers' association, a big part of the problem is the absence of "efficient management criteria."

Critics of the U.S. Postal Service, made independent by a 1970 postal reorganization law, say the new regime of the American mails has brought greater costs and reduced service, precisely the opposite effects of those the reform intended.

In Italy, however, the problem has an additional dimension. Not only have postal fees been kept artifically low at the political cost of a multimillion-dollar deficit, but for years the system has been filled largely by employes who obtained their jobs through political patronage.

Rather than hire new employes, the ministry until very recently preferred to help those already on the rolls by allowing them to work countless hours of tiring and counterproductive overtime.