Éamonn Fitzmaurice has detailed the contents of a letter one Kerry player received this summer which "crossed the line".
The player received an anonymous letter in which he was told "to jump off a cliff, and take three or four other players with him".
Fitzmaurice, who stepped down as Kerry manager on Saturday, said he's used to abusive letters since the start of his tenure in 2013, but this was too far.
"Something that changed this summer was players, and one player in particular, got a letter," he told Off The Ball's OTB AM.
"It was more that what was in the letter that annoyed me. I felt it was gone too far.
"When its coming at a manager, and the selectors got a few this summer as well, which was no harm for them to get a touch of it! But when it goes to a player, it's gone too far.
"The player was told in the letter to jump off a cliff and take three or four other players with him"
Eamonn Fitzmaurice reveals the sheer extent of the vitriol aimed at Kerry's playersAugust 7, 2018
"The lads are amateurs. There a good as bunch as you can meet. They're so dedicated and they're so committed to the Kerry jersey, it's unbelievable. I'd always defend them to a hilt."
He added: "I didn't tell that player that I was going to discuss this. I wouldn't like to name him. For the same player, it would be water of a duck's back. It didn't know a stir out of him, it didn't bother him.
"I thought it was poor form to be honest about it.
"That one to the player would cross the line.
A number of fellow inter-county managers have spoken about the abuse sent to them since Fitzmaurice first mentioned the "box full of anonymous letters" he has at home on Saturday.
Limerick manager John Kiely, who is preparing for an All-Ireland final, has a similar collection of abusive letters.
"I have a box at home too. I keep it in the box, it stays in the box. My wife picks it up and vets it and doesn’t let anything too serious come my way," he said.
Former Clare and Dublin manager Anthony Daly told the Irish Examiner PaperTalk GAA podcast he managed to ring a Clare fan sending abusive letters from Mayo.
"I looked up his name in the phone book, and found this uniquely Clare name and I rang him. I told him, 'If you ever send a letter to my house again, I'll personally drive to Mayo and meet you at your front door.'"
Former Mayo manager James Horan said he got the gardaí involved in some of the hate mail he received.
"Some of them would be very concerning without going into what they were. You needed to go to the appropriate authorities with it," he told Off The Ball.