If you are facing trouble in sending mails via email clients, like Outlook, just check the following:

1. Open a DOS/Shell prompt and execute the command: telnet 25 and telnet 26
Do replace with your domain name. If 25 is not responding while 26 is, then configure your email client to use port 25 for Outgoing mailserver port.

2. Ensure that "My Server requires authentication" is enabled

3. Disable "Use SSL"