Yes, you can use Windows 8.1 on all the versions of Windows released to date. I have also used their support section and checked for compatibility. It seems that IncrediMail is compatible with Windows versions up to Windows 10 (Windows 10 included as well).
Use the following URL to download the application: http://www.incredimail.com/
Note: When installing, make sure to run the setup as Administrator. This avoids possible installation errors.
