Convert Password-Protected PST Files to MBOX

[nyxamir]

To convert a password-protected PST file to MBOX, first make sure you can access the PST and its mailbox data. Once the file opens properly, select the emails or folders you want to move, choose MBOX as the output format, and save the converted files to your preferred location. Keep a copy of the original PST until you have checked the converted data.

I would suggest DataVare PST to MBOX Converter Expert for the conversion. It supports ANSI and Unicode PST files, lets you preview mailbox contents, and provides options to convert selected folders or the complete PST without requiring Microsoft Outlook.
You can try the free version with a sample PST first and check the MBOX output before converting the complete mailbox.