Dimenticavo, sto usando SMTP server Free. Infatti se lo chiudo il messaggio di errore cambia.

Warning: mail() [function.mail]: Failed to connect to mailserver at "localhost" port 25, verify your "SMTP" and "smtp_port" setting in php.ini or use ini_set() in C:\Programmi\AppServ\www\auor\scrivimi.php on line 4

Warning: Cannot modify header information - headers already sent by (output started at C:\Programmi\AppServ\www\auor\scrivimi.php:4) in C:\Programmi\AppServ\www\auor\scrivimi.php on line 5