The Englishman, 32, has created headlines throughout his career, including for serving a prison sentence in 2008 for common assault and affray.
Barton joined Marseille in August 2012, while still serving a 12-match ban for his sending off on the final day of the 2011-12 Premier League season while playing for QPR, and a subsequent altercation with City players Sergio Aguero and Vincent Kompany.
And he revealed on Wednesday that he had received a letter disapproving of his presence in Ligue 1, posting an image of what he labelled as 'fan mail' on his official Twitter page.
"A little friendly advice: as soon as you finish your season in your current club, I advice you to leave France immediatly," the error-strewn letter read.
"In fact, if you're not left France before June 1st, I will come to you specially Marseille smash your head shots baseball bat. And then I will finish and you'll die a knife open mouth once emptied your blood b******.
"Do you understand? You have until June 1st to leave France and return to your country mother*******."
Barton took the letter in good spirits, posting on Twitter: "Not sure what's more offensive, the grammar or the intent?"
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