The two most probable reasons:

1. You have not authenticated before the server. For security reasons, our servers require you to check your email before sending any messages. Please make sure that you have "SMTP Authentication" enabled in your email client. Here are tutorials on how to configure your accounts with OUTLOOK and EUDORA .

2. Your ISP blocks port 25. Here is an article on how to tell whether port 25 is blocked by your ISP. In short, if your ISP blocks port 25, you will have to ask them how to set your mail servers.