Tu forse usi questa:

DataInputStream in = new DataInputStream(mailSocket.getInputStream()); //Deprecato

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adesso:
InputStreamReader inr = new InputStreamReader(mailSocket.getInputStream());
BufferedReader in = new BufferedReader(inr);


Oppure in un unica riga!
BufferedReader in = new BufferedReader(new InputStreamReader(mailSocket.getInputStream()));


e poi:
String str = in.readLine();

Prova cosi
Fabio