.pst files are unique to MS Outlook and the format is, in fact, protected by Microsoft.
I doubt very much that there is a third party application (free or otherwise) available to replace Outlook AND use .pst files.
What is ransomware and how do I protect my PC from WannaCry?
Disney layout supervisor Rob Dressel on the challenges of visualising Moana
Siri vs Google Assistant
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.