How are the individual files named?

From the archive's own name plus a number, in the order the messages appear in the file — which is the order they arrived. That keeps the sequence intact and avoids the two ways message-derived names go wrong: subjects contain characters a filesystem refuses, and dozens of messages in one thread share a subject exactly.

Do the attachments come along?

Yes — each .eml is the complete message, attachments encoded inside it exactly as they were in the archive. That is why the split files add up to roughly the size of the archive rather than to something smaller. Extracting the attachments as separate files is a second step, and worth doing for the large ones.

Can I put the messages back into a mail client?

Yes. Most clients import .eml by dragging the files into a folder, and Thunderbird, Apple Mail and Outlook all accept them. What you will not get back automatically is the folder structure, since that lived in the account rather than in the messages — create the folders first and drop the right files into each.

Splitting a Mail Archive Into One File per Message

Published 9/8/2026 · 3 min read · File tools

Daniel Okonkwo — Front-end developer and tech writer at OneKitly

Web performance · File formats

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In short

Splitting an archive turns a single opaque object into a folder of ordinary files, and almost everything you want to do with old mail gets easier at that point. Your operating system's search indexes .eml files and does not index inside an .mbox; a mail client opens one on a double-click and cannot open the archive at all; a backup that copies changed files copies one small message rather than the whole multi-gigabyte archive every time you receive something. Deleting also becomes possible per message rather than all or nothing. The split reads the separator lines — From, a sender, a date — and writes one file per message, which you download together as a zip. Nothing is uploaded: the archive is read in your browser, which for a mailbox is the difference between a conversion and a disclosure.

An .mbox is one enormous file your operating system cannot search. Turn it into .eml files and every message becomes something your machine indexes, your mail client opens and you can delete one at a time.

One file per message is a filing system

Once messages are separate files, the tools you already have start working on them. Sorting by date, moving a year into its own folder, keeping the twenty that matter and deleting the rest, attaching one to another message, putting a few under version control with a project — none of that is possible while everything is glued into one archive, and all of it is ordinary once the archive is a folder.

What to keep, and for how long

An archive you cannot open is not an archive, it is a file you are afraid to delete. Splitting it lets you make an actual decision: keep the correspondence that documents something — a contract, a diagnosis, a dispute, a family exchange — and let the rest go. It also lets you extract the attachments once and store those where they belong, which is usually where the real value was hiding.

MBOX to EMLSplit an mbox export into one .eml file per message, ready to open individually.Try the tool →

Frequently asked questions

How are the individual files named?: From the archive's own name plus a number, in the order the messages appear in the file — which is the order they arrived. That keeps the sequence intact and avoids the two ways message-derived names go wrong: subjects contain characters a filesystem refuses, and dozens of messages in one thread share a subject exactly.
Do the attachments come along?: Yes — each .eml is the complete message, attachments encoded inside it exactly as they were in the archive. That is why the split files add up to roughly the size of the archive rather than to something smaller. Extracting the attachments as separate files is a second step, and worth doing for the large ones.
Can I put the messages back into a mail client?: Yes. Most clients import .eml by dragging the files into a folder, and Thunderbird, Apple Mail and Outlook all accept them. What you will not get back automatically is the folder structure, since that lived in the account rather than in the messages — create the folders first and drop the right files into each.

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ExplainerAn Attachment Grows by a Third Inside the EmailMail carries files as text, three bytes written as four characters. A 10 MiB attachment occupies 13.68 MiB of the message, which is why a 25 MB limit really means about 18 MiB of files.ExplainerAn .mbox Is Messages Glued End to End, and the Glue Is a LineOne file, thousands of messages, separated by a line beginning From. The format has one famous weakness, and it is the reason a message can lose its ending without anything reporting an error.How-toOpening an .eml File Without Telling the SenderA saved message is a text file you can read anywhere. The catch is that opening it in something that loads remote images announces the moment you did — which is what a tracking pixel is for.ExplainerEmail HTML Is Twenty Years Behind the Web, on PurposeNo stylesheet, no script, layout built from nested tables and styles written on every single tag. Knowing why explains what an exported HTML file will and will not look like.ExplainerWhy Your Subject Line Arrives as =?UTF-8?B? and a String of LettersMail headers are allowed to carry ASCII only, so an accented subject is encoded before it travels. Two encodings are in use, both start with =?, and decoding them is the whole difference between a readable message and a wall of symbols.How-toSplitting a File That Is Too Big to SendThe last resort when nothing can be compressed further. It works, and it has three sharp edges: the parts are useless on their own, the order is absolute, and the megabyte you set is not the megabyte the limit means.

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