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Installed "ImportExportTools" extension as instructed, but the "Import mbox file" option is grey i.e. not usable under menu "Tools/ImportExportTools".
Can anyone please tell me any other alternative to import mbox file? Thank you very much!

Questa recensione riguarda una versione precedente (3.2.4.1) del componente aggiuntivo. 

As it's written above,

"Note: if you want to import messages or MBOX files, you must first select a valid folder as a target, otherwise the import options are disabled"

Select "Local Folders" and the option will be active