the letter was from the president of Milan Club Palazzo Marino, Paolo Bianco, who started an initiative for fans to write emails addressed to Silvio regarding taking the club back in his hands.
Also, Berlusconi himself is honorary presiden of Milan Club Palazzo Marino. The email of Milan Club Palazzo Marino can be found in the first link:
http://www.calciomercato.com/index.php?c=12&a=25793
and
http://www.calciomercato.com/index.php?c=12&a=25863