While I do get your point, usually, these problems happen when the app developers don't keep up with updates. Like any other operating system, Windows 10 needs to evolve and for that to happen things need to change and feature needs to be deprecated (for security reasons, because the technology is old, or it's no longer effective), which may cause some outdated apps to spot working correctly.
You can use these steps to uninstall the Windows 10 Creators Update from your computer. However, you only have 10 days after installing the update to remove. Otherwise, the files to bring your system back to previous update will be automatically deleted, and you'll need to reinstall a previous version manually if you want to go back.
Have you tried reinstalling Eudora to see if that fixes the problem?