Hi, I would suggest using the best and most reliable solution available in the market to convert PST to MBOX file format without any hassle. Check out the outstanding DRS Outlook PST converter which can easily convert PST files to EML, MSG, HTML, MBOX, PDF, Gmail, AOL, Thunderbird, IMAP, Office 365, etc. There is also a free demo version to export the first 50 emails for free.
PST to MBOX convert manually:
Step 1: Open Microsoft Outlook.
Step 2: Go to File and click on Open & Export.
Step 3: Click the Import/Export button.
Step 4: Select Export to a file and click Next.
Step 5: Click on Personal Folder File(.pst) and then click on Next.
Step 6: Now in the Export Personal Folders, check if Include subfolders are selected. Hence all the files will be exported.
Step 7: Save the exported file and click on Finish.
Hope This Works,
Peter