Some ISPs block port 25 which is used for SMTP. To test this, you can proceed in several ways:
In Windows, go to the Start menu and then select "Run". Type: telnet domain.com 25 (where domain.com is replaced by your domain). An ms-dos pop-up should appear with an Exim (the software used by our email servers) welcome message.
If no message appears, it is possible that your ISP has blocked port 25. If your ISP has indeed blocked port 25, we can overcome this problem by allowing another port to be used for SMTP (usually port 26) . To check if the server is configured for this, follow the same steps once more and enter "telnet domain.com 26" (without the quotes).
If there is still no welcome message, open a support ticket and we will open port 26 on your server or investigate the cause of the problem. You will then be asked to change the SMTP port in the email software. Please consult the documentation attached to the software for this.