A jealous woman told her ex's new partner they were still enjoying "unbelievably fantastic" sex in an affair before murdering her love rival with "demonic savagery" a court heard.
Spurned Sarah Williams, 35, sent the "spiteful" letter to Sadie Hartley, 60, after her ex Ian Johnston, 57, began a new relationship with the older woman.
But after the letter failed to break up the couple, "obsessed" Williams attacked businesswoman Ms Hartley with "demonic savagery", Preston Crown Court heard.
Williams paralysed her victim with a stun gun before stabbing her 40 times in an "orgy of violence" on the doorstep of her £500,000 home in the upmarket village of Helmshore, Lancashire.
One blow with the kitchen knife used by Williams went straight through her victim's neck, the court heard.
Mother-of-two Ms Hartley a "decent, hard-working" communications director, was found dead in a pool of blood in the hallway of her house the day after the attack on January 14 this year.
Ski travel firm employee Williams recruited her friend Katrina Walsh, 56, a horse riding instructor, to help with her "murderous mission" the jury heard.
Both deny murder.
Williams had plotted the murder for 17 months, after ex-fireman Mr Johnston broke off their relationship when she became "possessive and difficult".
On the second day of the trial John McDermott QC, prosecuting, told the court of a letter sent by Williams in September 2014, telling Ms Hartley how she was having an affair with her man.
The letter read: "Dear Sadie
"I think you should know that Ian has been cheating on you for over a year.
"He's been having an affair with me since returning from Camp Suisse in August 2013.
"The sex is unbelievably fantastic, the best he's ever had by a really, really long way. We have never been able to get enough of each other.
"It satisfied a need in him he will never really be able to suppress or manage without.
"Ian is stressed out and extremely depressed. His mental state is somewhat of a serious concern to me hence while I'm writing this.
"I feel you have played a significant part in getting him in the state he's in now, which appears to be worsening by the day.
"Despite doing everything you have done so far, to buy him and trap him, he's still lying to you, cheating and sleeping with me behind your back.
"Maybe it is about time that somebody told you.
"Probably it just goes to show you can buy and trap someone with money and blackmail but can't make them love you or be faithful to you.
"Should you choose to talk to him about this bear in mind it was not a one off, an accident or mistake or any other form of excuse.
"This was a choice made freely, over and over and over again for now more than 12 months because it was what he wanted to do.
"Clearly these are not the actions of someone with any respect, desire, love or affection for you whatsoever.
"Should you wish to discuss anything here you are free to contact me."
Mr McDermott told the jury: "You will decide whether that was an act of kindness to warn her about the perfidious man who she was with, or whether it was an act of extreme spite."