Maggs, 31, posted a photo of the letter to his Facebook page 16 minutes after he was fired.
"Here's the integrity of Lenny Gramkow," Maggs wrote.
The letter has since been shared hundreds of times and spawned anger from across the country.
"At least we know your county made the right choice and I hope the best for your family," a woman named Deb Cortez wrote on Maggs' Facebook page. "I don't live nowhere around you, but you have my support."
“What a sorry, pitiful, vindictive loser,” Sally Hoff wrote. “Congratulations, Mark!”
Another commenter, Leslee Carroll, found humor in the situation.
"And in all CAPS!" Carroll wrote. "He's super angry … lol."
Maggs told The Argus Leader that he wasn't surprised by his termination, since the topic had come up when Gramkow first found out his employee was seeking to knock him out of his job.
Gramkow was within his rights to do so. The Associated Press pointed out that South Dakota is an "employment-at-will" state, which means employers can fire their workers without cause, though there are some exceptions.
The news was still hard to take, the sheriff-elect said.
"When I got back to my election party and told my wife, she was very emotional," Maggs said. "It hit her hard. We knew that meant coming at the end of the month, we'd be losing health insurance."
Maggs, who said he ran for sheriff to improve services to county taxpayers, received 73 percent of the vote in the election. He takes office in January.
The Bon Homme Sheriff's Office is made up of the sheriff, two deputies and five dispatchers who also serve as jailers, The Argus Leader reported. The agency serves a population of about 7,000 people, according to U.S. Census Bureau statistics.
The county's small government has been inundated with phone calls from people protesting Maggs' firing. The Sheriff's Office's Facebook page has also been filled with angry comments criticizing Gramkow.
Maggs asked people to calm down.
"We're going to be fine," he said of his family. "Stop calling the Sheriff's Office. Those guys still have a job to do."
In the meantime, the Bon Homme County Commission has set up a special meeting for next week to discuss the situation. Maggs told the newspaper that commissioners, who he spoke with Thursday, have promised to help him figure out a plan.
As of Friday morning, a Change.org petition demanding that Maggs be reinstated until he takes over as sheriff had amassed more than 1,800 signatures.