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juangra - in Windows,
- In Control Panel->All Control Panel Items->Programs and Features, check that you have the Kindle Preview software installed
- In Control Panel->All Control Panel Items->Default Programs->Set Associations, check that the Kindle Preview software is the default handler for ePubs
In Calibre
- Check Preferences->Behaviour->Use internal viewer for->EPUB, is unchecked
That should result in ePubs always opening in the Kindle Preview software when an ePub is viewed in calibre.
BR