You may receive this error in your email client settings at some point. In Outlook this is a bug and can usually be resolved by restarting your email client and making sure you have SMTP authentication enabled.

This is not a server problem and is caused by a misconfigured email client, ISP's blocking access to port 25 and using port 26 instead or a DNS problem with your ISP.

To confirm that the mail server is online and responding you can perform the following:

From the 'Start' button click 'Run'
Type in 'telnet yourdomain.com 25'
You should receive something to the effect of:

220-server.dnsprotect.com ESMTP Exim x.xx #1 day, xx Month 2004 time
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.

This confirms there are *no problems* with the SMTP server
To confirm the POP server is online and responding:

From the 'Start' button click 'Run'
Type in 'telnet yourdomain.com 110'
You should receive something to the effect of:

+OK POP3 [cppop x.x] at [xx.xx.xx.xx] (yourdomain.com)

This confirms the POP server is online and there are *no problems* with the POP server.

Link to Microsoft support: http://support.microsoft.com/default.aspx?scid=kb;en-us;q295164