Yahoo is actively blocking email notifiers and generates a bogus error message when it detects its service is being accessed through such a notifier. When you ask Yahoo support about this error, this is their response:
We're sorry you've had difficulty accessing Yahoo!. The error you received is typically caused by unusual activity from either your computer or your Internet Service Provider (ISP) to Yahoo!. While this type of problem is usually temporary, if you continue to experience this error, we recommend that you try the following suggestions:
- Contact your ISP and let them know you are experiencing connectivity problems with Yahoo!. You may have difficulty accessing Yahoo! if we detect unusual network traffic coming from your ISP. Only your ISP will be able to address and resolve this abnormal activity.
- Check that you are not using any third party software program or application to access Yahoo! services. Please keep in mind that these types of programs are not supported by Yahoo! and may violate our Terms of Service, and therefore may contribute to your receiving this error.
- If you have not scanned your computer for spyware and viruses, please do so now. Some spyware programs and computer viruses will run separate programs on your computer for a variety of purposes, sometimes slowing your computer down in the process, and even preventing you from accessing certain Internet sites such as Yahoo!.
- You may want to try accessing Yahoo! through a different computer or through a different Internet connection.
There you have it. Yahoo blocks "third party software" from accessing their services.