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 usually not a server problem, and is caused by one of the following errors.

1. Your email client is misconfigured
2. Your ISP is blocking access to port 25
3. There is a DNS problem with your ISP
4. There is an error with SMTP on one of our servers

Below you will find the most common solutions to fix the above problems:

1. Shutdown and restart your computer.
2. Make sure Outlook is configured for "SMTP: My server requires authentication," and then use the same username and password as POP3 account.
3. Use port 26 for SMTP instead of port 25. Many ISPs are now blocking port 25 to prevent their clients from spamming.
4. Temporarly turn off all virus and/or firewall software.

If all of the above solutions fail, please confirm that the mail server is online and responding you can perform the following:

1. From the Start button click Run
2. Type in telnet yourdomain.com 25

You should receive something to the effect of:

220-opteron3.crucialwebhost.com ESMTP Exim 4.52 #1 Day, XX Month Year HH:MM:SS
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* connecting with the SMTP server. For more information, please see the Microsoft support article.