So here we go.
here is what I found: from here I understood what are these markers and why they are there.
Then here on SO I have found a solution that was working for me...at the beginning. After a further check, I have found some email that was not correctly deecoded by the decodeText method, and then I have found why: sometime, actually I don't know why, subjects came surrounded by quotes, and each time it happened the decodeText did not work.
Workaround: simply remove all quotes from string (or the quotes at the beginning and at the end if you can have more into your string):
string2beConverted = string2beConverted.replace("\"", ""); String decodedString = MimeUtiliy.decodeText(string2beConverted);
Cristiano