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An MSG file is a single email message saved from Microsoft Outlook — complete with headers, body text, formatting, embedded images, and attachments. It's a binary format based on Microsoft's Compound File Binary Format (the same OLE2 structure used by old .doc files), which makes it compact but proprietary.
The format is ubiquitous in corporate environments where Outlook dominates. Legal discovery, compliance archiving, and business record-keeping all produce MSG files. They preserve the complete email exactly as it appeared in Outlook, including reply chains, read receipts, and custom properties.
Outside Outlook, opening MSG is inconvenient. Microsoft Outlook (paid) reads them natively. Free options include MSG Viewer (Windows), Thunderbird with a plugin, or online viewers. For universal access, convert to EML (standard email format) or PDF. The content is the same — only the packaging is proprietary.