Kesha's Mom Threatens Producer

Kesha's mom sent Dr. Luke's lawyer an email a year ago ... alleging Luke raped her daughter -- but there's a distinct smell of extortion.

In the email -- obtained by TMZ -- Pebe Sebert complains that Luke screwed her out of music publishing rights connected to her daughter. She then says.

"We, me and Kesha and her friends, like Lady Gaga, are going to make all of this Really PUBLIC, in the next few days. Luke date raped Kesha when she was 18. Nicky Hilton's birthday? Paris Hilton's house? Luke gave Kesha pills. She ended up naked in his hotel room 2 days later. No longer a virgin?"

The email then gives an ultimatum:

"Do we all want this to come out? Either Luke releases Kesha from all legal contracts, and gives me back all my publishing, or we, Kesha and I, tell the truth."

And then there's this:

"And if I wind up dead, TRUST ME, IT'S IN WRITING TO CALL YOU AND LUKE."

Luke says in his lawsuit against Kesha her allegations are a pack of lies and designed to scare him into releasing her from a binding recording contract. He also claims Kesha and her mom have admitted they lied about the allegations.

Kesha has sued Luke for sexual, physical and emotional abuse.