Tiny Online Tools
What an MBOX is
An MBOX file is the oldest and plainest way to store many messages: they are simply concatenated, each introduced by a line that begins From — with a space, not a colon. That single convention is the entire format. Gmail Takeout, Thunderbird, mutt and almost every mailing-list archive produce one.
The listing is built from headers alone. Each message's header block is read up to its first blank line and no further, which keeps the work proportional to header bytes rather than to the archive: a 5.3 MB, 673-message developer-list archive lists in well under a tenth of a second. A message is parsed in full only when you select it.
The dialect question, honestly
A message body may itself contain a line starting From , and the format has no way to distinguish that from the start of the next message. Writers escape it as >From — and here the dialects part company. mboxo escapes only From , so a body line that genuinely began >From is corrupted on the way out. mboxrd escapes >*From as well, which makes the escaping reversible.
The two are usually indistinguishable from the file. An archive containing >>From anywhere is definitely mboxrd; one containing >From and nothing doubled is consistent with both. That case is reported as ambiguous rather than guessed at, and you can override the choice. The setting only changes body text — a leading > that is either stripped or kept — so if a quoted line looks wrong, try the other mode.
What you can take away
Any message can be saved as a standalone .eml, byte-identical to its slice of the archive after unescaping. The listing itself exports as CSV or JSON, filtered exactly as you see it, which is a practical way to inventory a large archive before deciding what to keep.
The archive never leaves your browser. HTML bodies render inside a sandboxed frame with remote images blocked by default, so opening an old newsletter does not tell anyone you did.