There is a possibility that your ISP is blocking port 25. Providers like AOL block this to prevent outside users from relaying bulk email off their servers. You should contact your provider to see if port 25 is blocked. Otherwise you have incorrectly entered details in your email client for the outgoing server (SMTP) for example make sure you are authenticating the outgoing mail server as described in other support database articles, or software on your machine like anti-virus, firewall, Norton, Symantec, etc is conflicting. If port 25 is blocked, you could try using port 587 instead.