EML vs MSG: what is the difference?

An .eml file is an email in the format email itself uses: the exact text a mail server delivered, with the headers, the body and every attachment in one file that any mail client can open. An .msgfile is Microsoft Outlook’s format for one saved item: a small binary container holding Outlook’s own properties for that message.

Both hold a single message. What differs is who can open them, and how much of the message as it was originally sent survives inside.

What is an EML file?

An .eml is a single message saved exactly as it travelled between mail servers. Open one in a text editor and you can read it: a block of header lines — From, To, Subject, Date, and the Received lines each server added on the way — then a blank line, then the body. Attachments and HTML versions of the body are packed in as MIME parts, with binary files encoded as base64 text.

Because it is the internet standard, nearly every mail client reads and writes it. Thunderbird and Apple Mail save messages as .eml, and so does Gmail’s “Download message”. Apple Mail also keeps its own copies as .emlx, which is the same message with a length line in front and a list of Apple Mail flags after it.

To open one, double-click it and your default mail client should show it. If you have no mail client on that computer, or would rather not open an unexpected message in the one that is signed in as you, the EML viewer reads it in your browser with every remote image blocked, and EML to PDF turns it into a document you can attach to anything.

What is an MSG file?

An .msgis how classic Outlook for Windows saves a single item when you use Save As or drag a message out of Outlook onto the desktop. It is not an email in the internet’s format at all. It is a Microsoft Compound File — a miniature filesystem inside one file — holding a set of Outlook properties: the subject, the sender, the recipients, the body, each attachment, and Outlook-only details such as categories and follow-up flags. The same format also stores meetings, contacts and tasks.

That is why an .msg does nothing when double-clicked on a Mac, and why renaming one to .emlproduces gibberish. Whether the original internet headers are inside depends on where the message came from: mail Outlook received usually carries them, while a message composed and saved locally has only Outlook’s own properties.

To open one without Outlook, use the MSG viewer, which reads the Compound File directly in your browser on any operating system. To turn it into something other programs open, convert it with MSG to EML or MSG to PDF.

Should you save an email as EML or MSG?

Choose EML when the message needs to open anywhere, or when it may be needed later as a record. The file is the message as delivered, so its headers show where it came from and when, and its DKIM signature can still be checked against it. It is also plain text, which makes it easy to archive and search.

Choose MSG when the file is going to another Outlook user and Outlook-specific details matter to them, such as categories, follow-up flags or a meeting request they need to respond to. Classic Outlook for Windows has no .eml option under Save As, so if that is what you use and you need an .eml, save the message as .msg and convert it afterwards.

Converting between them

MSG to EML is a reconstruction rather than a copy, because the .msg holds properties rather than the original message. When the .msgstored the original header block, the result is very close to the message as it arrived; when it did not, the headers are rebuilt from Outlook’s properties and the output says so. Bodies, inline images and attachments come across either way. MSG to EML does this in your browser and re-reads every file it produces before offering it.

EML to MSG is not offered here. Writing a file that Outlook opens reliably can only be confirmed by testing it in Outlook, and a converter that produced files Outlook quietly mangled would be worse than none. Formats and limits explains the reasoning.

And MBOX?

An .mbox is a whole folder of messages in one file: each message in the same text format as an .eml, one after another, separated by a line beginning From . Google Takeout delivers Gmail this way. Thunderbird keeps each mail folder as an mbox file with no extension, beside an .msfindex file. Apple Mail’s Export Mailbox writes a folder ending in .mbox, with the mailbox itself in a file called mbox inside it.

The MBOX viewer opens one in your browser, lets you search it by sender, subject and date, and exports the messages you pick as .eml files, a new .mbox, a single PDF or their attachments.

Outlook’s whole-mailbox files, .pst and .ost, are a different and much more complex format. They are not supported here, and Formats and limits says why.