We presume that you are facing the error when you are trying to send mails using your email client.
This error can be caused by one or more of the following:
1. The firewall or antivirus running in your system.
2. A misconfiguration with your email client.
3. ISP’s blocking access to port 25
4. A DNS problem with your ISP.
Please try these fixes for this problem:
1. Shutdown and restart your PC.
2. Be sure to configure Outlook (If you are using it as your email client) for “SMTP: My server requires authentication”, and you can select to use the same username and password as pop3.
3. Use port 26 for SMTP instead of port 25. Many ISPs are now blocking port 25, to prevent their clients from spamming. They will often make this change without warning you they are going to do so.
4. Temporarily turn off Virus and/or firewall software.
Many common errors in outlook and their possible solutions are available here.
If you still have the same problem, please open a support ticket so that we can check the issue in detail. Also provide us the telnet and traceroute results.