Raymond John Brookes, who has been in custody for three months, mailed a four-page document to Sydney's largest mosque in Lakemba in January.
He also sent the screed to independent Western Australia senator Fatima Payman, pleading guilty to four counts of sending a document threatening to kill.
Titled "V for Vendetta" the often rambling diatribe called for revenge against Muslims, Aboriginal people and Torres Strait Islanders, Labor, the Greens and journalists he described as "socialist".
Brookes appeared by video link from Silverwater prison wearing prison-issued greens during a hearing at Bankstown Local Court on Tuesday.
The isolated 70-year-old gorged on a steady diet of extremist views fuelled by alcohol and compounded by tenancy issues and disabilities, his lawyer told the court.
"A combination of things made him the perfect person to become brainwashed by certain types of media or ... propaganda of some sorts," his lawyer Danilo Rajkovic said.
"He had been isolated for several months, he was drinking himself to sleep most of the days, he was watching Sky News and getting on YouTube.
"He fell into some type of internet rabbit hole."
Mr Rajkovic said the letters contained a mangled mix of left-wing and right-wing ideologies.
The Belmore man called for anyone who protested during Invasion Day rallies to be "beaten to death" in the street and demanded a ban on protests and immigration.
Judge George Breton chided Brookes for his "disgraceful attitude" before handing down an intensive correction order for 15 months with strict supervision and no alcohol consumption.
Brookes had been in custody since January 24.
Judge Breton said the man's "rantings and ravings" demonstrated hatred for society.
"Inciting hatred and violence ... seems to be escalating in our world," the judge said, referring to the Bondi attack in December and current geopolitical conflicts as examples.
"People like you Mr Brookes are filling (the world) with more hate."
Two of the letters Brookes sent were to Lakemba mosque - the spiritual home of Australia's Muslim community.
One was addressed to "The Mullah", erroneously referring to the top Muslim cleric, and the other to the Australian Muslim Advocacy Network (AMAN).
The mailing address for the advocacy group was incorrect, but it said it had received racist vitriol emboldened by political rhetoric about Muslims in recent years.
"It intensifies with the political environment, and right now it is especially intense," an AMAN spokesperson told AAP.
"Politicians need to stop collectively blaming our community and religion. They are the biggest risk to our community safety."
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