SMTP error - failed to set sender

[Clikti]

Hello.

My postfix + roundcube webmail server work good still a couple of hours now ...
I can't send an email anymore
I got this error in my log file

[06-May-2011 15:47:32] Invalid response code received from server (452):
[06-May-2011 15:47:32 +0200]: SMTP Error: SMTP error: Failed to set sender 'user@mydomain' in /var/www/webmail/program/steps/mail/func.inc on line 1497 (POST /webmail/?_unlock=loading1304689601238?_task=mail&_action=send)

Thanks for helping me

[SKaero]

Enable smtp_debug and post the log.