This could be caused by your ISP (Internet Service Provider) having port 25 blocked. There are some ISP's that will block port 25 to stop spam from being sent off their mail servers. In this case you will need to contact your ISP, verify that they have port 25 blocked, and request your username and password for your SMTP with them. In your e-mail client you will need to have a check mark in "My outgoing server (SMTP) requires authentication" and the "Log-on using" selected. This is where you will enter the SMTP username and password that your ISP provided for you.