MSG
Microsoft Outlook's proprietary binary format for a single email message, storing headers, body, and attachments in a Compound Document Structure. Unlike EML, MSG is not a plain-text standard.
MSG is a binary file format developed by Microsoft and used by Outlook on Windows. It is based on the Compound Document Structure (OLE2), the same container technology used by legacy Office documents. A single MSG file encapsulates all parts of a message: headers, plain-text and HTML body, embedded images, and attachments — but in a proprietary binary layout rather than MIME text.
Because MSG is not an open standard, reading MSG files requires either Outlook itself or a library that reverse-engineers the format. This limits portability: MSG files do not open natively in Thunderbird, Apple Mail, or most Unix mail tools without conversion. Exporting Outlook mail to EML or MBOX before archiving gives better long-term accessibility.
Users migrating away from Windows or Outlook often convert MSG files to EML or MBOX as a first step. Once converted, the messages can be opened in cross-platform tools and searched without needing Microsoft software installed.
Related terms
A single-message file in MIME format, containing headers, body, and attachments. EML files are widely supported across email clients and are useful for archiving or sharing individual messages.
Personal Storage Table — Microsoft Outlook's container file for an entire mailbox, including folders, messages, contacts, and calendar items. Also known as OST when used for offline sync with Exchange or Microsoft 365.