"All my years being in the league, I never received more emotional mail from people than I did about that issue," Mara told The MMQB. "'If any of your players ever do that, we are never coming to another Giants game.' It wasn't one or two letters. It was a lot. It's an emotional, emotional issue for a lot of people, moreso than any other issue I've run into."
Mara made his comments in a story that was published Thursday on the website, but the remarks only became widely circulated Monday when they were highlighted in another story on the site.
Wide receiver Torrey Smith, currently with the Philadelphia Eagles but a teammate of Kaepernick's for the past two seasons, took offense to Mara's comments in a series of tweets Monday.
The Giants re-signed Josh Brown last year despite domestic violence allegations against the kicker, and Mara said in August that the team was aware of the allegations and "comfortable" with the facts when they agreed to his return. The Giants released Brown in October after he admitted abusing his then-wife to the team.
Mara told The MMQB that the Giants never discussed signing Kaepernick this offseason.
Kaepernick has remained unsigned this offseason but recently made a visit to the Seattle Seahawks. Around 50 people stood outside the league's offices in New York last week in a show of support for the quarterback.
Kaepernick sat during "The Star-Spangled Banner" in an Aug. 26 preseason game to protest racial inequality in the United States. He then altered his method of protest on Sept. 1 by kneeling during the national anthem and continued to do so throughout the season.
Sources have told ESPN's Adam Schefter that Kaepernick will stand for the national anthem this season as he no longer wants his method of protest to detract from the positive change he believes has been created.