[exim] incomplete transaction ?? :)

i have problem with one mail account
i can not send email through this account & receive.

logs :

2004-09-02 16:44:59 H=aae150.internetdsl.tpnet.pl (Grego) [83.16.4.150] F=<euroduct@???> rejected RCPT <magafik@???>: Sender verify failed
2004-09-02 16:45:00 H=aae150.internetdsl.tpnet.pl (Grego) [83.16.4.150] incomplete transaction (connection lost) from <euroduct@???>

while trying to send email using this pop3 account :
2004-09-02 16:54:24 H=aae150.internetdsl.tpnet.pl (Grego) [83.16.4.150] F=<euroduct@???> rejected RCPT <magafik@???>: Sender verify failed

dont know where to start for looking for a solution
any clues ?