[newbie] mail in kmail

[Oder]

I seem to be unable to send mail to [EMAIL PROTECTED] list in Linux box; only in windows. Every time in kmail I received - Sending failed: a SMTP error occurred. Command: RCPT Response: 550 5.7.1 [EMAIL PROTECTED]... Relaying denied. Proper authentication required. Does anyone could me help? Thanks in advance, Oder.