First, restart your computer and verify you have the correct settings for your mail program as many mail programs cache old settings. Our email tutorials are located here. The username is email@example.com and the password you defined when creating the email account in the control panel. For sending and receiving the same username and password are used. If everything checks out open a command prompt/shell with your operating system (Windows for example) and type "telnet mail.yourdomain.com 25". If you are unable to connect, your internet service provider is blocking you from sending mail outside their service. For such cases we make port 50 available to you as an alternative port for sending mail via SMTP.
Also make sure you have authentication enabled for outgoing email/SMTP. Those credentials will be the same as your other settings for the mailbox. In some cases try turning TLS on or off.