mail() problem

Sorry to bring it up again but I really cannot find the problem. PHP4 on Unix/Apache mail() used to work fine one day and next day it didnt work anymore for any files on the server, sendmail is fine, php.ini is default to sendmail -t I am really running out of options here, can someone point me into the right directions, maybe what setting I should check regarding php and apache that could affect mail() or sendmail(which works fine). mail() doesnt report any errors but doesnt send email either. Oleg