Microsoft Outlook Email storage .pst file is a proprietary format. Surprisingly no other email client can import the pst file itself right away. 2 best way I found to backup Microsoft Outlook Email stores in .pst file.
Both methods require Microsoft Outlook installed locally on the PC.
(A) Backup Outlook email to GMail.
1. Setup Gmail IMAP inside Outlook.
2. Transfer email from Outlook to Gmail. It takes long time to move email 1 by 1. But once it is done, you realize all the emails with related topic are threaded together in Gmail way. And it is very easy to search your email inside GMail.
(B) Backup Outlook to email Thunderbird.