MBOX Splitter

About this tool

An mbox file is one long text file holding many emails end to end. Mail clients and export services — Thunderbird, Apple Mail, Gmail Takeout, mailing-list archives — all hand you a single .mbox, which is awkward when you only want one message, or when the thing you need to open the mail with expects one file per message. This tool splits the archive back into the individual messages it contains, each a ready-to-save .eml with a suggested filename.

Messages are separated the classic way: a From postmark line at the start of a line. The space after From is what distinguishes the separator from a From: header, so header lines never split a message by mistake. Each message is then sliced out verbatim — headers, MIME structure, and base64 attachments are copied byte for byte, never re-serialized — so what you save is a faithful copy of the original mail. The postmark line itself is dropped by default, because a .eml file is a bare RFC 5322 message; turn on Keep the From postmark line if you want to reassemble an mbox later.

Four things you can ask for:

• Every message as .eml text — each message under a ===== 001-name.eml (N bytes) ===== header, so you can copy one out or save the whole thing.
• Index of what is inside — a numbered table of filename, date, sender, subject, and size, with no bodies. Useful for checking an archive before doing anything with it.
• JSON records — { index, filename, subject, from, date, bytes, eml } per message, for scripting.
• One raw message only — combine with a message number to get exactly one .eml and nothing else. This is the mode to use with the Download link.

Filenames follow whichever scheme you pick — numbered, subject, date, or Message-ID — and always keep the 001- index prefix so the original archive order and filename uniqueness survive. Subjects written as RFC 2047 encoded words (=?utf-8?q?Caf=C3=A9?=) are decoded first and then slugged to portable lowercase ASCII.

Exporters escape body lines that begin with From by writing >From (the mboxo/mboxrd convention). Undo >From body quoting is on by default so bodies match the message as sent; switch it off if you want the archive bytes exactly as they were stored.

Everything runs locally in your browser as WebAssembly — the archive is never uploaded.

Limits: one run splits at most 2000 messages, and input arrives as text through the field above, so a multi-gigabyte export should be cut down first. The tool returns text, not a ZIP of files: to save a single message, choose One raw message only with a message number and use the Download link, then rename the downloaded file to .eml.

Worked example

Input:

From [email protected] Mon Sep 03 10:00:00 2018
From: Alice <[email protected]>
Subject: Quarterly report
Message-ID: <[email protected]>
Date: Mon, 3 Sep 2018 10:00:00 +0000

Numbers attached.

From [email protected] Mon Sep 03 11:30:00 2018
From: Bob <[email protected]>
Subject: Lunch?
Message-ID: <[email protected]>
Date: Mon, 3 Sep 2018 11:30:00 +0000

One o'clock?

With Index of what is inside and date filenames, the output is:

2 message(s)

  1. 001-2018-09-03-1000.eml
     date:    2018-09-03T10:00:00+00:00
     from:    [email protected]
     subject: Quarterly report
     size:    141 bytes

  2. 002-2018-09-03-1130.eml
     date:    2018-09-03T11:30:00+00:00
     from:    [email protected]
     subject: Lunch?
     size:    124 bytes

Switching to One raw message only with message 2 returns Bob's message on its own, starting at From: Bob <[email protected]> — no postmark, no other mail.

FAQ