EML vs MSG, One Holds What Travelled And One Holds More

An EML file carries the .eml extension and an MSG file the .msg. Both hold one message. What separates them is what they are a copy of. An EML records a message as it crossed the network. An MSG records a message as Outlook holds it, which includes a great deal that never crossed anything.

That difference decides everything else, including the one thing worth knowing before you save a folder of either. Moving between them is not symmetrical. Everything in an EML file fits inside an MSG file. Not everything in an MSG file has anywhere to go in an EML file.

EML Files And MSG Files Copy Two Different Things

A message that reaches you crossed a network to get there. Only things that could survive that journey are in it. Headers, a body, attachments turned into characters. Nothing else could have travelled.

Outlook is not restricted that way. It keeps its own properties on every message, whether or not that message ever moved.

This is why the comparison is not about which is better. One is a faithful record of a message. The other keeps that same record and adds everything Outlook thought about it. Both are complete, at different jobs.

The EML to MSG Asymmetry Nobody Mentions

Because one is a superset of the other, the two directions behave nothing alike.

headers      kept
body         kept
attachments  kept
Outlook bits  none to
             begin with

headers      kept
body         kept
attachments  kept
flags        no field
categories   no field
voting       no field

Categories are the ones people miss. An archive where somebody colour coded three thousand messages by client loses every one of those labels. The loss shows up months later, when a search that used to work returns nothing. Exporting the category list separately costs a minute and is almost never done.

What an EML File And an MSG File Cost to Open

The second difference matters more day to day. It also runs the opposite way.

So the format that holds more is also the format that fewer things can read. That trade is the whole decision. It is not close once you know who opens the file next.

A saved MSG reaching somebody on a Mac is the common failure. They receive a file their mail program cannot open and cannot identify, from a colleague who saw nothing wrong at their end. The sender had Outlook. That was the only reason it looked fine.

How an EML File And an MSG File Store the Same Attachment

Both keep attachments. Neither keeps them the same way, which explains their sizes.

In an EML file
  Content-Type: application/pdf
  Content-Transfer-Encoding: base64
  JVBERi0xLjQKJcfsj6IKNSAwIG9iago...
  the file as characters, a third larger

In an MSG file
  a stream inside the compound file
  the file as bytes, unchanged
  named by a property rather than a header

An EML has to encode everything into characters because it was built to cross a channel that only carries text. An MSG never faced that constraint, so it keeps the bytes as they are.

Neither format is reliably smaller. The encoding costs an EML file about a third on its attachments. The container structure costs an MSG file on overhead and on the properties it carries. Which wins depends on the message rather than on the format, so file size tells you nothing useful about which one you are holding.

Which to Save, Decided by Who Opens It

1. For yourself, staying in Outlook. MSG. It keeps everything, opens instantly and loses nothing, because the thing that reads it is the thing that wrote it.
2. For a colleague, on a machine you cannot see. EML. The chance they have Outlook is not worth the chance they do not.
3. For an archive nobody will touch for years. EML. A published standard outlives a vendor container. The reader in ten years is a person you will never meet.
4. For a legal or compliance request. Ask them. Where nobody specifies, EML, for the same reason as the archive.
5. Where the Outlook layer is the point. MSG, with a word to whoever receives it. If categories and flags are what somebody actually needs, no other single message format holds them.

Why This Question Comes Up at All

Neither format is one anybody chooses in the ordinary way. Both arrive because somebody dragged a message out of a mail program onto a desktop. Which one landed depended entirely on which program it was.

That is why folders of these turn up mixed. It also explains why the question gets asked after the fact rather than before. Somebody inherits four hundred saved messages, half opening cleanly and half not, split along a line nobody drew on purpose.

Mixed folders are the normal case, not the odd one. Sort by extension before deciding anything. The EML half needs no work at all. The MSG half is where every question you have actually applies.

The MSG half needs software that understands the container, which is not the same as software that understands mail. Our file viewers handle it in a browser tab. Where a mixed folder has to become one consistent archive, the converters take it from there.