The 37-year-old, who became the first Black woman to play for England's cricket team in 2001, shared a picture of the letter on Twitter Wednesday, during a week when the sport's issues with racism have been laid bare.
Rainford-Brent captioned her tweet: "Interesting... Born in south London but apparently I was found naked in Africa as a primitive. Had some letters in my time but this one up there!"
The letter contained a number of racist and sexist slurs and referred multiple times to "White culture."
It comes after former cricket player Azeem Rafiq gave testimony describing a pattern of racist and discriminatory behavior within English cricket, speaking to UK lawmakers on Tuesday.
Throughout the hearing, Rafiq shared his experience of being bullied at Yorkshire County Cricket Club (YCCC) and broke down in tears on more than one occasion.
In a summary of a report produced by an independent panel for the YCCC, the panel concluded that "there is no question" that Rafiq "was the victim of racial harassment."
Rainford-Brent represented her country 29 times until 2010 before moving into sports broadcasting.
In 2020, she partnered with cricket legend Michael Holding to produce a powerful short film to address the issue of racism in both cricket and society following the police murder of George Floyd.
"I grew up in a very multicultural, diverse London with all sorts of colors -- Black, White, Asian," she said in the Sky Sports video.
"Everyone was a melting pot, and I noticed that as soon as I walked into the world of cricket, the comments started.
"I had comments about where I grew up, and the fact that I had a long name, maybe my mum didn't know who my dad was; about my hair, body parts, especially the derriere shall we say; about the food that I ate and that it stank.
"I question myself why I stayed so long. I love the game, it has so much more to offer, but it can be really difficult dealing with that day in and day out."