What is an EML file?

One email saved to disk — headers, body and attachments in a single file that any program can read.

Look inside your own EML file

Drop it into the viewer and see the headers, the message and the attachments laid out. Nothing is uploaded.

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Or read what the format is first.

The short answer

An .eml file is a single email message saved as a file. Its structure is defined by RFC 5322, the standard that describes what an internet message looks like, with attachments encoded using MIME.

It is plain text. Open one in a text editor and you will see the headers at the top, a blank line, then the body — although the body is usually Base64 or quoted-printable encoded, which is why it looks like gibberish.

What is inside

  • Headers — From, To, Subject, Date, and dozens of technical ones recording every server the message passed through.
  • Body — often twice: a plain-text version and an HTML version, so that any program can display something.
  • Attachments — encoded as text and separated by boundary markers, which is why a 1 MB attachment makes a 1.4 MB file.
  • Authentication results — the SPF, DKIM and DMARC outcomes recorded by the server that accepted the message.

Where these files come from

They appear whenever a message leaves its mailbox: dragging an email out of Thunderbird or Apple Mail onto the desktop, exporting from a webmail client, downloading evidence from a legal hold, or receiving one as an attachment because somebody forwarded a message as a file rather than inline.

That last case is the most common reason people meet the format and have no way to open it.

EML, MSG and MBOX

  • .eml — one message, open standard, readable by anything.
  • .msg — one message, Microsoft's own binary format, produced by Outlook.
  • .mbox — a whole mailbox: many messages concatenated into one large file.

If a file is a few kilobytes it is almost certainly a single message; if it is hundreds of megabytes, it is a mailbox.

Frequently asked questions

Is an EML file safe to open?
The file itself is text and cannot run. The risk, as with any email, lies in its attachments and links — opening the message to inspect it is the safe way to decide.
Can I rename a .msg to .eml?
No. They are different formats, not different extensions for the same thing. Renaming produces a file that nothing can read.
Why is my EML file bigger than the attachment it contains?
Attachments are encoded as text inside the file, which adds roughly a third to their size.
Can I read one in Notepad?
You will see the headers clearly. The body is usually encoded, so it appears as blocks of letters and digits rather than readable text.

Related guides

Open your EML file The viewer: drop the file and read it in seconds How to open an EML file The practical steps, with and without a mail client EML vs MSG Two formats for one message, and how to tell them apart

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