I inherited mail archives from an old university Unix server. The admin calls them Berkeley mailbox files, single large files with no extension, one per folder.
Outlook obviously has no import option for these. What is the sane path to get them into a PST?
Berkeley mail is the classic mbox format: every message concatenated into one plain text file, each starting with a From line. Outlook cannot read mbox at all, so the job is an mbox to PST conversion.
Direct route: a converter such as SysTools MBOX Converter reads the raw files and writes a PST. Point it at the folder of files, it detects them as mbox even without extensions, keeps the folder names and exports one PST you import via File, Open and Export.
Free route if it is a one time job: install Thunderbird, add the ImportExportTools NG add-on, import each mbox file into a local folder, then set up an IMAP account that Outlook also uses and drag the mail across through the server. It works but attachments heavy archives crawl and folder structure needs manual rebuilding.
Whichever route you take, check the biggest file first. Decades old mbox files sometimes have corrupted From separators that split one message into fragments. If the message count after conversion looks inflated, that is why.
Went with the converter route for the 14 files. Message counts match the old pine client totals so the separators held up fine.