This usually means there is a problem with your DNN settings.  Here are a few things to check:

Under Host Settings

1. Make sure you have a valid email address specified for the host email address.
2. Expand Advanced Settings then Expand SMTP Settings
3. Click the test link and see if you get an email.
4. If you don't you need to adjust your SMTP settings.  The SMTP settings will depend on your environment.  If you are running locally you want to trying copying the settings that you use in your mail client.

User Settings

1. Check the Host Account - Make sure it has a valid email address
2. Check the Admin Account - Make sure it has a valid email address.