What is an EML file?

EML is a plain-text format you can open in any editor. It is used for email and archiving.

The extension is .eml, and the full name is Email Message. Both matter less than what the file can hold, which is what the rest of this page is about.

It dates from 1982. The specification is RFC 5322.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

Mozilla Thunderbird, Microsoft Outlook and Apple Mail read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

EML is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

Almost nobody creates an EML file on purpose. They appear when a message leaves a mail client: drag an email out of Thunderbird or Apple Mail onto the desktop and what lands there is an EML. Ask Google for a copy of your data and the mailbox arrives as MBOX, but individual messages saved from the web interface come out as EML. Export from a webmail provider, a helpdesk system or an archiving product and EML is usually one of the options.

The second source is disclosure. When a company has to hand over correspondence — to a court, a regulator, an auditor, an employee exercising a data request — EML is the format that gets sent, because it is the message exactly as it was transmitted rather than somebody’s rendering of it.

Open an EML in a text editor and the top of it is readable straight away: From, To, Subject, Date, and then a long tail of headers that mail servers added on the way — Received lines tracing each hop, message identifiers, authentication results, spam scores. That trail is the reason the format matters in a dispute. It is evidence of how the message travelled, and it is not present in a screenshot or a printout.

Below the headers the body is split into MIME parts: usually a plain-text version and an HTML version of the same message, so that any client can show something. Attachments follow as further parts, encoded in base64 — which is why a file containing a two-megabyte photograph is about 2.7 megabytes on disk and looks, in an editor, like a wall of meaningless characters. Nothing is compressed, and nothing is hidden.

This is the practical problem that brings most people to the format. Double-clicking an EML on a Windows machine with no mail program set up produces a dialog asking which application to use, and the honest answer is that there may not be one installed.

Three ways out. Thunderbird is free, reads EML natively and is the least trouble if you have several to look at. Any text editor will show you the headers and the plain-text body, which is enough to answer "who sent this and when" without installing anything. And if the file has to be filed, forwarded or attached to something, converting it to PDF is the option that removes the dependency entirely — the recipient needs nothing but a browser.

MSG is Outlook’s own format and is a Microsoft compound-document structure rather than text. It carries things that only make sense inside Outlook — categories, flags, the state of the message in the mailbox — and it needs Outlook, or something that has reverse-engineered it, to be read at all.

EML is the open equivalent, defined by RFC 5322 and readable by anything. If you were given an EML, somebody exported from a standards-based client or a server; if you were given an MSG, it came out of Outlook. The contents are broadly the same message; the difference is who can open it in ten years without a licence.

Formats that survive decades have two properties: they are plain text, and they are specified in a public document that has not moved. EML has both. The relevant standard was first published in 1982 and its current revision is still freely readable, which is why a message written in 1995 opens today with no special software.

It also stores the message rather than a picture of it. A PDF of an email is a rendering — one client’s idea of how the HTML should look, on one day, at one window width. The EML is the source that rendering came from, complete with the headers that show it is genuine. Keep the EML as the record and generate the PDF whenever somebody needs to read it.

Attachments are the usual complaint. They are inside the file, but a text editor shows them as base64 and only a mail client or a converter will separate them back out. If you need the attachment and not the message, open the EML in Thunderbird and save it from there.

Character encoding is the second. Older messages declare encodings that modern editors guess wrong, which is how a German email ends up full of question marks. The declaration is in the headers, so the information is there — it is the editor, not the file, that is losing it.

The third is that Windows has no built-in handler. A file that will not open is nearly always this rather than a corrupted message, and it is worth checking before concluding the export failed.

A full mailbox export arrives as MBOX — one file with every message concatenated inside it, which is convenient for a backup and useless for handing over three specific emails. Splitting it into individual EML files is the normal first step in any disclosure exercise.

The order matters if you are working to a deadline: get the MBOX split first, identify the messages that are actually relevant, then convert only those to PDF for the bundle. Converting the whole mailbox first is how people end up with four thousand PDFs and no way to find anything.