full story and photos: http://news.bbc.co.uk/go/em/fr/-/2/hi/uk_news/scotland/7861259.stm
Former armed forces corporal John Hopkins and his severely ill wife Ann became afraid to leave their home after they received the letter two weeks ago.
Ann, 54, who suffers from an inoperable brain tumour, collapsed when she opened the envelope, which was addressed to her.
Police were immediately called to the couple's home in Closeburn, Dumfries, and took the unsigned letter away. An investigation has been launched to try to trace its author.
The letter warned that Mr Hopkins would "suffer the consequences" of an unexplained accident.
Mr Hopkins, 57, who became an estate manager after a career in the Royal Corps of Transport (RCT), is his wife's full-time carer.
He said: "Ever since this letter arrived, my wife and I have been looking over our shoulders. We are much more aware of our surroundings now."
Mr Hopkins is in dispute with his former employers, Defence Deer Management (DDM), which look after deer on the Ministry of Defence's (MoD) land.
He held the voluntary role of assistant principal deer manager at RAF Spadeadam in Cumbria for eight years but says he was sacked last year after he made a series of complaints about alleged poaching incidents and malpractice by MoD staff.
He said: "I felt the way the deer were being run on the estate was in some ways substandard and I believed that some were being poached illegally, and so I made a lot of noise about it. Now I feel like I'm being persecuted for it.
"Many people know me within DDM, and they know about my wife too and how ill she is. For someone to send this threat, and address it to her, is really beyond the pale. I'm sickened by the whole thing, and so is everyone else I have spoken to about it."
The letter warns him to "cease meddling in DDM affairs" or else he would "suffer the consequences".
Mr Hopkins claims he was fired unfairly from his role and was not given the right to a hearing or an appeal.
He added: "I am fighting to get my position back, but I didn't expect something as malicious as this to happen."
DDM, as part of Defence Estates, looks after the deer in all of the MoD's land across the UK, covering hundreds of thousands of hectares, and is answerable to the MoD.
The workforce of more than 350 is made up of volunteers from the armed forces and retired MoD employees who ensure that deer populations remain healthy and in balance.
There is no evidence that the threat against Mr Hopkins came from an MoD or Defence Estates employee.
A spokesman from Dumfries and Galloway Constabulary said: "We can confirm that we got a report that a member of the public received a threatening letter on Friday 16 January.
"While no specific detail was described in the letter and police believe there is no immediate threat to the individuals involved, safety advice has been issued to the recipient of the letter. Police are investigating the circumstances behind the letter and enquiries are continuing into the source of its author.
"Any member of the public who receives any form of bogus or threatening mail is to contact police or any police officer. Never reply and never give out personal details."
An MoD spokesman said: "The Ministry of Defence is aware of a complaint made to Dumfries and Galloway Constabulary relating to a former volunteer member of the Defence Deer Management organisation. We understand that police enquiries are ongoing and will of course co-operate fully with those enquiries."