The two most probable reasons are:
1. Your e-mail client is not configured to authenticate itself 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 and visit the guidelines for configuring Outlook Express, Mozilla Thunderbird and Eudora.
2. Your ISP blocks port 25.
In case your ISP is filtering the default SMTP email port 25, you can use an alternative 2525 port, which we have opened for you. In your email client configuration you should choose 2525 as a default SMTP port and you will be able to send your messages.
Here is an article on how to tell whether port 25 is blocked by your ISP.
