MSG file
- MSG file
A single Outlook item — usually an email, but possibly a calendar entry, contact or task — saved in Microsoft’s own binary container format rather than in the open format used by internet mail.
Not just an email in a different wrapper
.msg files come from the Microsoft compound-file format, a structured binary
container that stores an Outlook item as a set of typed properties. That is a
genuinely different thing from an .eml, which is plain
text you can read with any editor.
The difference is not gratuitous. .msg can represent things internet mail has
no field for: Exchange sender identities distinct from SMTP addresses, voting
button responses, follow-up flags, categories, and non-mail items like meetings
and tasks. Outlook needed somewhere to put them, and internet mail has nowhere.
Where you meet them
Anywhere the classic Outlook desktop client is in use, which in practice means
most law firms. Dragging a message from Outlook to a Windows folder produces a
.msg. So does File then Save As. So does forwarding a message as an
attachment.
The new Outlook for Windows and Outlook on the web behave differently — both
are built on the web client and download messages as .eml — which is the
detail that makes most .msg problems solvable without a format conversion.
Converting Outlook email covers the routes.
What converting one costs
Going from .msg to .eml drops the properties that internet mail cannot
represent. Body, header fields, and attachments all survive; Exchange-specific
metadata does not, because there is nowhere for it to go.
That loss is usually immaterial — none of it was going to appear on an exhibit
— and occasionally decisive, when a case turns on Outlook-specific behaviour
such as a disputed meeting response. In that situation the .msg natives
themselves are what gets produced, and any converted copy is a courtesy.