The bizarre scheme had been going on for two years, with the letters sent to high profile figures from the worlds of sport, TV, business and politics.
The Milan branch of Digos, Italy's anti-terrorism squad, tracked down the pair and arrested them on Wednesday for threats and defamation. The senders were a 71-year-old former police officer and his partner, aged 51, Ansa reported.
"The man followed the news closely and was very up-to-date on things," Digos officer Carmine Mele told the news agency. "He had sent the letters practically everywhere."
They sent threatening messages to the directors of Copenhagen Zoo, teachers accused of child abuse, football clubs and politicians, and many others. Many of the envelopes included toilet paper soiled with their pet dog's faeces.
A notebook found in the couple's home contained names and addresses of the unlucky recipients - along with the reason for targeting them.
As shown in a photo of the book shared by police, names were divided into categories, including "murderer" "paedophile" and "corruption". A raid on the couple's house reportedly revealed 110 more letters, all addressed, stamped and ready to be sent.
A page from the notebook found in the couple's home. Photo: Italian police
Digos officers were able to track down the pair after observing the letters all shared some similar characteristics: black ink, cursive writing, and stamps from foreign countries (most often Azerbaijan, Bhuta and Zaire). The letters were then traced to Milan.
Oolice caught the two in the act, posting yet another letter in a letterbox in the Basiglio district of the northern city - as shown in the police video below.