The envelope was sent to the Ministry earlier this week while Kotzias' aides also received threatening calls, Greek national news agency AMNA reported.
"All those who will sign the sell out of our Macedonia will face the elimination of their entire families," read the letter.
A police investigation was underway to find the perpetrators. Kotzias had received two similar letters in early February with threats against him and his family.
The long-standing dispute between the two neighbours started when Macedonia declared independence from former Yugoslavia in 1991.
Macedonia has annoyed Greece, which has a northern province bearing the same name. Athens is worried that Macedonia's identical name with its province could lead to territorial disputes.
As talks mediated by the UN aimed to resolve the issue have stepped up in recent months, both sides have expressed hope that a positive outcome could come within the first half of 2018.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
