The quantity involved can be staggering. At the height of his fame, Johnny Depp was said to receive up to 10,000 letters a week.
The dawn of the digital age - in which public figures with a Twitter account can be messaged directly - has made the process easier. The White House says it processes 20,000 messages addressed to President Barack Obama each day.
Some celebrities have given up on fan mail altogether. In 2008, Beatles drummer Ringo Starr announced he would be throwing it all out in the future, because he had "too much to do".
Others do attempt to get through it themselves. Robert Pattinson, star of the Twilight films, claimed in an interview that he read "tonnes and tonnes" of letters from fans.
Most, however, appear to conclude that acknowledging correspondence from fans is part of the cost of fame. As a result, they often outsource the task of opening, reading and replying to correspondence.
Sylvia "Spanky" Taylor, 58, has run a fan-mail answering service, in Burbank, California, since 1987. She and her six staff process up to 20,000 items of mail a month on behalf of 26 celebrities. Over the years, her clients have included Depp, Rob Lowe and Michael J Fox.
Typically, correspondence is acknowledged by a fan letter and a photo with a printed "autograph".
Television actors tend to generate more mail than film stars, Taylor says, because "if they're in your home every Sunday evening, you feel much more familiar with them".
Most letters from fans are simply declarations of affection and admiration, she adds. A few beg for money. A small number contain disturbing material or threats which require her to contact the celebrity's security team and law enforcement.
You get quite a dismal view of the world sometimes”
The biggest logistical headache for Taylor is working out how to dispose of the correspondence that passes through her office - including gifts like chocolate and stuffed animals.
While some celebrities do like to go through their mail personally - Fox, she says, always made an effort to read as many as possible - the majority simply do not have time.
"With candy, it gets thrown out - I don't know if someone's stuck a needle in it," she says.
Presents such as soft toys are distributed around local hospitals, she adds. As for the letters, most "just get shredded and recycled".
To most disinterested observers, this may be unsurprising.
But the fate of their correspondence is something most committed fans will not wish to dwell on, says Lynn Zubernis, an expert in the psychology of fandom at West Chester University.
While the relationship between the fan and the celebrity may exist only in the mind of the former, it stems from a deeply-rooted human need for community and belonging, Zubernis believes.
As a result, even receiving a mass-produced letter of acknowledgement and a photo stamped with a reproduced signature can be a powerful experience.
"People have a tremendous need to connect with the person they are idolising," she says. "They can't ring them up and say, 'Can we have coffee?'
"It's not about the autograph. It's about the moment of connection."
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By Jon Kelly