Grammys Dress Code -- Celebrities Warned ... Keep Your Genitals Covered!!!

CBS doesn't want to see a SINGLE celebrity butt crack at the Grammys on Sunday ... and no boobs or genitals either ... this according to a warning the network fired off to all "talent" appearing on the show.

The email -- which surfaced online -- was sent by CBS Standards and Practices to people connected to the celebrities who are expected to appear on camera during the broadcast ... and the network makes it clear, it's not taking any chances with the FCC this year.

"Please be sure that buttocks and female breasts are adequately covered," the letter reads.

"Thong type costumes are problematic. Please avoid exposing bare fleshy under curves of the buttocks and buttock crack."

It goes on ... "Bare sides or under curvature of the breasts is also problematic. Please avoid sheer see-through clothing that could possibly expose female breast nipples."

"Please be sure the genital region is adequately covered so that there is no visible 'puffy' bare skin exposure. Please avoid commercial identification of actual brand name products on T-shirts."

"Foreign language on wardrobe will need to be cleared. OBSCENITY OR PARTIALLY SEEN OBSCENITY ON WARDROBE IS UNACCEPTABLE FOR BROADCAST. This as well, pertains to audience members that appear on camera.'

So, a star can wear a pink ribbon ... as long as it doesn't say "The American Breast Cancer Foundation."

We reached out to CBS -- so far, no word back.