Yesderday i was sending mail, and all of the sudden, i got this message back from the administrator mailbox " The message reached the recipient's e-mail system, but delivery was refused. Attempt to resend the message. If it still fails, contact your system administrator. #5.3.0". The mail server was working fine for the whole day. I got the error by the end of the day. I didn't installed anything an no one worked on the server. The server is very slow and there's a lot of system errors. I did reboot the server and still got the erros.