What Is a MSG File? The Outlook Email Format, Explained

MSG files turn up when someone drags an email out of Outlook onto their desktop, when a legal team exports a mailbox for review, or when a ticketing system archives correspondence. They are ordinary emails, but they are stored in a way that makes them stubborn: unreadable in a text editor, unopenable in Gmail, and awkward to search. Understanding what is inside one explains every bit of that behavior, and it tells you what you can and cannot get back out.

What is a MSG file?

A MSG file is a single Microsoft Outlook item saved to disk as its own file. Most often it is an email, but Outlook uses the same format for calendar appointments, contacts, and tasks. It contains the sender, every recipient, the date, the subject, the message body in both plain text and HTML, any attachments, and a set of Outlook-specific properties.

The file extension is .msg. It is Microsoft's own format, not an internet standard, which is the root of most of the friction people run into with it.

What is inside a MSG file?

Inside, a .msg is not text at all. It is a Compound File Binary Format document, sometimes called OLE compound storage: a small hierarchical filesystem packed into one file, with folders and streams instead of lines. Each part of the email lives in its own stream, and each stream is labeled with a MAPI property tag, which is Microsoft's numeric code for what the value means.

So the subject is not stored on a line beginning with "Subject:". It sits in a stream tagged with the property code for subject, encoded as UTF-16, which is why a text editor shows a letter, then a blank, then a letter. Attachments live in their own sub-storages, complete with filename, content type, and the file bytes themselves. Everything is there. It is just addressed by code rather than by name.

Both formats store a single message. Only one of them travels well between systems.

Why is a MSG file not just text?

Because Outlook stores more than the email. Alongside the standard message, a .msg carries Outlook's own bookkeeping: read state, flags, categories, conversation identifiers, and the RTF body Outlook historically used. Plain MIME has nowhere to put those, so Microsoft chose a container that could hold arbitrary typed properties.

That decision buys fidelity inside the Microsoft world and costs portability everywhere else. It is the same trade you see with .doc against .txt.

Where do MSG files come from?

You get a .msg whenever a message leaves Outlook as a file. Dragging an email from the message list to a folder produces one. So does File, then Save As, with Outlook Message Format selected. Exports from ediscovery platforms, archiving tools, and case management systems frequently produce folders of them, one file per message, often thousands at a time.

This is why MSG files usually arrive in bulk rather than alone. Someone has exported a mailbox, and now there is a directory of cryptically named files and a question about what to do with them.

How do I open a MSG file?

Double-click it and Outlook opens it. Without Outlook, use a standalone MSG viewer, an online viewer, or convert the file to .eml and open it in Windows Mail, Thunderbird, or Apple Mail. Renaming the extension from .msg to .eml does not work, because the contents are binary and a mail client will read them as broken text.

The full set of methods, including the Mac and Gmail cases, is laid out in the guide on how to open a MSG file.

Can I open a MSG file in Excel?

Not directly. Excel has no importer for the format, and opening a .msg through Excel gives you binary noise rather than columns. To see MSG data in a spreadsheet, parse the files first: read each message, pull out the fields you want, and write them as rows.

That is a genuine conversion, not a rename. The MSG to Excel converter reads .msg files in your browser and gives you a sheet with sender, recipients, date, subject, and body in their own columns, plus any custom field you name. It works on a single file or a folder of thousands.

What is the difference between MSG and EML?

MSG is Outlook's binary format and EML is the plain-text internet standard. Both store one email. EML opens in nearly any mail client and can be read in a text editor; MSG expects Outlook or a dedicated viewer, and it preserves Outlook-only properties that EML has no place for.

Which one you want depends on where the message is going. For long-term archives that other people will need to open, EML is the safer bet. Our comparison of EML vs MSG files goes through the trade-offs, and what an EML file is covers the other side in the same detail as this page.

Can I delete MSG files?

Deleting a .msg deletes that saved copy of the email, not the message in your mailbox. If the email still sits in Outlook on the server, removing the exported file changes nothing. If the .msg is the only remaining copy, for example after a mailbox was decommissioned, then it is your record and deleting it loses the email.

Before clearing out a folder of exports, check whether anyone is relying on them as the retained copy. For regulated records, that decision usually belongs to whoever owns your retention policy, not to whoever needs the disk space.

How do I search inside hundreds of MSG files?

Windows Search indexes .msg contents only when Outlook is installed, because the indexer relies on Outlook's filter to read the format. Without it, searching a folder of MSG files returns nothing useful. That surprises people mid-project, usually on a machine that was set up without Office.

The durable fix is to convert the messages into something searchable. Parse the folder once into a CSV or an Excel workbook with one row per email, and every subject, sender, date, and body becomes filterable in a tool you already know. It also gives you something you can hand to a colleague who does not have a viewer. If the documents attached to those emails are scans rather than text, a folder of images needs document text recognition before any of it becomes searchable.

Do I need Outlook to work with MSG files?

To open one comfortably, Outlook helps. To read one, a free viewer is enough. To get the data out of many of them, you need neither: a parser reads the binary structure directly and writes the fields to a spreadsheet, which is usually what people wanted in the first place when they went looking for a way to open the files.

Where to start

If you have one .msg and you just want to read it, open it in Outlook or a viewer. If you have a folder of them and a deadline, do not open them one by one. Parse the folder into a spreadsheet, then work in the spreadsheet: sort by sender, filter by date, search the bodies, and pull the invoice numbers or case references you were hunting for. That turns an archive you cannot open into data you can use.