The 35-year-old singer had approached the 'Devil Wears Prada' star to get involved with an idea she had for her latest album, 'Meaning of Life', but even though her idol was unable to take part, the 'Breakaway' hitmaker was bowled over by the kind response she received.
She told Refinery 29: ''Recently -- I am a huge Meryl Streep fan, OK? I mean, who isn't? But she's one of my favourite humans on the planet.
''I came up with this idea to release my album and have people I like in the limelight do a little something for it. So I sent her this request, but I don't know her at all, so I sent it like through her people... channels... I don't even know how it got to her, but magically it did! And she totally emailed me back!
''It was the greatest rejection I've ever had. She couldn't do it because she's filming a movie somewhere, but she wrote back the kindest, nicest note, and I was floored because a lot of times people just say, 'We asked her people and they said she can't because she's busy.'
''But she literally sent, it said 'From Meryl.' You would've thought I was opening a letter from Harvard to see if I got accepted! I was freaking out!''
Kelly - who has children River, three, and 18-month-old Remington with husband Brandon Blackstock - is happy to be a role model for body positivity but she doesn't want her appearance to become a bigger topic of conversation than her music.
She said: ''I don't mind it at all. My only thing is that I don't want my body to be the topic all the time.
''Because it's like man, you work so hard on something, you don't want people to just focus on your body...
''The reason I do talk about it is because I want people to know that we all are different, and it's about what makes you happy. If you're unhappy, then change it, but don't let anyone project any unhappiness onto you.
''So I don't mind carrying the body positivity flag, but I do want people to remember that I'm carrying another flag, too: my music!''
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