EML file viewer

A .eml file is one email saved verbatim: the headers, then a blank line, then the body, exactly as it travelled. Mail clients export them when you drag a message to the desktop or forward one as an attachment, which is why they turn up in bug reports and spam complaints.

The body is rarely a single thing. A typical message is a multipart container holding a plain text version and an HTML version of the same content, and often a third part with the images the HTML refers to. Each part carries its own encoding, usually base64 or quoted-printable, and its own character set. That is why opening a .eml in a text editor shows walls of base64 rather than the message: the file is correct, it is just not meant to be read directly.

Most .eml files people open are not their own mail. They are complaints a recipient forwarded, or a message someone attached to a support ticket. Rendering the remote images in one fetches them from the sender's servers, which fires any tracking pixels the message carries and reports back that it was opened, along with the IP address that opened it.

Embedded images are different and load immediately: those are parts of the file itself, referenced with cid: URLs, so showing them reaches the network for nothing. Anything pointing at a URL stays blocked until you turn it on, and the tool says how many it held back.

Encrypted bodies, whether S/MIME or PGP, are ciphertext without the private key, and no browser tool can change that. TNEF attachments, the winmail.dat files older Outlook servers produce, are a proprietary container this does not unpack.

When a message is simply malformed, which is common in mail generated by scripts, the viewer degrades rather than failing: the headers and the raw source are always shown, and anything that could not be parsed appears as a note rather than an empty screen.