Open MBOX files, even huge ones.

An MBOX file is a mailbox: many emails stored one after another in a single file. Gmail Takeout, Thunderbird and Apple Mail use it to export or store mail.

Unzip the Takeout download, find the .mbox file in the Mail folder and drop it on the viewer above. Every message is listed, newest first, and you can search and read them.

Yes. The viewer reads only the start of each message to list it and the full message only when you open it, so files of many gigabytes open on an ordinary computer. Listing a very large mailbox takes a little while the first time.

In your Thunderbird profile, under Mail or ImapMail. Each folder is a file with no extension, such as Inbox or Sent, next to an .msf file. Drop the file without the extension.

No. The engine runs inside your browser tab and reads the file from your disk. Nothing is sent to a server, which matters for a whole mailbox.

Not yet. Takeout stores each message once with its labels in a header, and the viewer lists every message in one list. Search by subject or sender to find what you need.

Yes. MBOX to PST converts the mailbox into a PST file that Outlook can open, and checks every message before you save it.