If you are still having problems after making sure that your email setup is configured properly, your Internet Service Provider (ISP) may not allow smtp relaying. ISPs nationwide are currently closing down outbound port 25 for their customers to prevent spam from originating from their IP range.
Microsoft also has a knowledgebase entry for its customers about smtp relay blocking:
You can test if port 25 is opened by your ISP by running this command in a telnet session:
telnet yourdomain.com 25
If you can not connect or if you are in doubt on how to do the telnet session, your best option will still be to contact your ISP.
ISPs usually offer one of the following solutions to their customers:
They open port 25 for you on request for your user ID.
They open port 26 for you to send mail. In this case, you would need to change your mail configuration to port 26 where applicable. Your ISP should be able to assist you with that, as they were the ones who changed important configuration settings on their end.
If they refuse to open port 25 and port 26, you might still be able to use a secure connection which runs on ports different than 25 and 26. Please ask your ISP to guide you to connect to a regular secure SMTP server.
Some ISPs changed their policies to only allow to send email through their mail servers. In that case, your only option will be to send email for your domain through webmail.
You can alternatively change your smtp setting to whatever your ISP specifies, i.e. mail.earthlink.net for EarthLink customers.