Causes:

1) You are not connected to the Internet or a network

2) Wrong setting in POP3  or SMTP fields.

3) DNS server error

3) Domain Name expired

Resolution:

1) Contact your ISP to fix the internet problem

2) Check your setting in Outlook / Outlook express.
     Any spelling mistake cause error on receiving or sending email.
     pop3: mail.yourdomain.com
     smtp: smtp.yourdomain.com or your ISP's smtp

3) Contact your DNS service provider to check for your domain pointing status.

4) Contact your Domain Registrar to renew your domain name.