The Portuguese outfit have labelled Bernardo Silva a "convicted racist", blasted Fernandinho for "lacking class or insight" and insisted Pep Guardiola's side are "lucky".
The damning newsletter has taken the war of words between the two clubs to a new level and is understood to have left Etihad bosses seething.
It comes in the wake of two clashes between City and Porto in the Champions League group stages that has resulted in bad blood between the clubs.
Guardiola has clashed with Port rival Sérgio Conceição several times following City's 3-2 win in Manchester in October.
The row continued in the wake of Tuesday's goalless draw in Porto that saw both sides qualify for the knockout stages.
Guardiola accused the home side of negative tactics, with Conceicao hitting back to claim City were a waste of money when he said: “If I had his budget, his players, and still couldn't get a win, I'd be sad too.”
Now Porto have slammed City with a newsletter, which singles out Silva for special attention following his one-game ban for posting a racist tweet last year.
In November 2019, Silva was banned and fined £50,000 after an FA commission accepted he 'did not intend' a tweet about team-mate Benjamin Mendy to be 'racist'.
Silva had compared Mendy to the character on a packet of Conguitos - a chocolate brand available in Spain and Portugal.
Get the latest transfer news straight into your inbox!
Want to be on the ball with all of the latest football news?
Well then sign up for the brilliant new Daily Star Sport email newsletter!
From the latest transfer news to the agenda-setting stories, get it all in your email inbox.
How do you sign up?
It only takes a matter of seconds.
Simply type your email address into the box at the top of this article and hit 'subscribe'.
And that's it, job done. You'll receive an email with all of the top news stories every single morning.
You can find out more information on our email newsletter on this link here.
The Porto newsletter has dragged the matter up again and described him as someone "known internationally for having been convicted of racism".
It also laid into Fernandinho and added: "There should have been a red card given to Fernandinho, who despite being 35 is a clear example that football money counts a lot, but it doesn't buy class or notion."