The 1961 record showcased a gritty, pleading vocal by 15-year-old Gladys Horton, who would remain with the group until the late '60s, when she left to care for her disabled son.
Horton died Wednesday in Sherman Oaks at age 65, said her other son, Vaughn Thornton. She had been in declining health for some time.
"Gladys was a very, very special lady, and I loved the way she sang with her raspy, soulful voice," Motown founder Berry Gordy said in a statement. "We will all miss her, and she will always be a part of the Motown family."
The Marvelettes never topped the chart again, but they had six Top 20 singles, including "Beachwood 4-5789," "Don't Mess With Bill" and "Playboy."
The Beatles recorded a version of "Please Mr. Postman" that was on their second U.S. album, and the Carpenters brought the song back to No.1 with their 1975 remake.
The original group — five students at Detroit's Inkster High School — was given a Motown audition after a school talent contest. After signing, they were asked to come up with an original song, so member Georgia Dobbins reworked a blues song that she got from a friend. Others, including Motown producers Robert Bateman and Brian Holland, also have been listed as co-writers.
With its rough edges and aggressive attitude, "Postman" sounded more like a product of the era's girl-group genre than the Motown hit machine that would soon develop, and the Marvelettes' prime period predated Motown's famous "charm school" grooming process.
As unpolished as they were, they were the first queens at the label, a position that led to some conflicts with their rivals. In one famous incident, the Supremes' Diana Ross punctuated a bout of bickering with Horton by driving a car in her direction and screeching to a stop with little room to spare.
In the mid-'60s, Smokey Robinson stepped in to produce the group, and member Wanda Young replaced Horton as lead singer for such softer, more sophisticated fare as "The Hunter Gets Captured by the Game."
The Marvelettes struggled with personal conflicts among the members, and as the Supremes and Vandellas ascended, Motown gave them less attention. When Horton gave birth to her son Sammie, who has cerebral palsy, she left the business to care for him.
"I didn't want to travel," she said in a 1985 interview with The Times. "I had to spend my time caring for my son. I'm an orphan, so I don't have any family I could leave him with while I was carrying on with my singing career."
Horton, who moved to the Los Angeles area, performed periodically in the 1980s and '90s, a challenge complicated by legal restrictions on the use of the Marvelettes' name.
Despite the ups and downs in her personal and professional life, she remained positive, said Ron Brewington of the Los Angeles chapter of the Motown Alumni Assn.
"I never heard her say anything about frustration," he said Thursday. "It was always 'Sammie, Sammie, Sammie. What does he need? What's he doing? What can I do for him?' That's a mother, a real mother....
"She loved people, loved to sing. That was her pride and joy was to sing. She just loved to sing and to make people happy."
Horton, who was born May 30, 1945, is survived by her sons, Sammie Coleman and Vaughn Thornton; and two grandchildren.
Plans for services were incomplete.