Causes for problems when sending e-mail messages are mainly two:
The first problem is related to inactivated option for authentication to the SMTP server. To activate this option in your e-mail client, search for “My server uses Authentication” in Microsoft Outlook and Outgoing Server (SMTP) – “Use name and password” in Mozilla Thunderbird.
The second problem is when your Internet Service Provider filters port 25. This port is necessary for you to contact our e-mail servers. In this case you should change the port from 25 to 26 in your e-mail client.