The short answer
- MSG is an Outlook item file format that stores a single email, appointment, contact or task, and opening it in new Outlook or Outlook on the web requires an eligible Microsoft 365 subscription.
- In new Outlook for Windows, drag the MSG file into the reading pane, double-click it, or right-click it and select Open with. In Outlook on the web, drag it into the reading pane. Both documented opening routes require an account with an eligible Microsoft 365 subscription. Microsoft does not document a native Mac or mobile opening procedure in its MSG opening instructions, or say whether the browser method works offline or processes the file only on your computer.
- MSG can contain attachments. Microsoft identifies attachments as a common route for computer viruses and blocks some attachment types, including certain executable files. The medium risk rating reflects potential risks from enclosed attachments. A file opening successfully, or passing structural length checks, does not establish that its attachments are safe.
MSG files: opening Outlook items and converting messages
Microsoft's MSG format saves one Outlook item in the file system. It uses Compound File Binary version 3, with hierarchical storage objects and streams. The top level represents the entire Message object; recipients and attachments have separate storage objects. String properties use either Unicode or non-Unicode forms, without mixing both in one file. The Library of Congress lists application/vnd.ms-outlook as its media type and notes that it is not registered with IANA.
For an email opened in classic Outlook, use Microsoft Print to PDF to create a PDF, or save the message as HTML to retain formatting, images, links and From, To, Cc and Subject information. Plain text loses bold, italics, font colors and inline image display. Keep the original MSG if you need its full item properties and attachments; a PDF or HTML copy is for reading only.
How to open it
In new Outlook for Windows, drag the MSG file into the reading pane, double-click it, or right-click it and select Open with. In Outlook on the web, drag it into the reading pane. Both documented opening routes require an account with an eligible Microsoft 365 subscription. Microsoft does not document a native Mac or mobile opening procedure in its MSG opening instructions, or say whether the browser method works offline or processes the file only on your computer.
In new Outlook for Windows, drag the MSG file into the reading pane, double-click it, or right-click it and select Open with. In Outlook on the web, drag it into the reading pane. Both documented opening routes require an account with an eligible Microsoft 365 subscription. Microsoft does not document a native Mac or mobile opening procedure in its MSG opening instructions, or say whether the browser method works offline or processes the file only on your computer.
For an email opened in classic Outlook, use Microsoft Print to PDF to create a PDF, or save the message as HTML to retain formatting, images, links and From, To, Cc and Subject information. Plain text loses bold, italics, font colors and inline image display. Keep the original MSG if you need its full item properties and attachments; a PDF or HTML copy is for reading only.
File safety
MSG can contain attachments. Microsoft identifies attachments as a common route for computer viruses and blocks some attachment types, including certain executable files. The medium risk rating reflects potential risks from enclosed attachments. A file opening successfully, or passing structural length checks, does not establish that its attachments are safe.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
For an email opened in classic Outlook, use Microsoft Print to PDF to create a PDF, or save the message as HTML to retain formatting, images, links and From, To, Cc and Subject information. Plain text loses bold, italics, font colors and inline image display. Keep the original MSG if you need its full item properties and attachments; a PDF or HTML copy is for reading only.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is an MSG file used for?
An MSG file stores a single Outlook item: an email, appointment, contact or task. An email MSG can include message properties, recipient information and attachment objects.
How do I open an MSG file on Windows in new Outlook?
Drag the file into new Outlook's reading pane, double-click it, or right-click and select Open with. Opening MSG in new Outlook requires an account with an eligible Microsoft 365 subscription.
How do I open an MSG file in a browser without desktop Outlook?
Open Outlook on the web and drag the MSG file into the reading pane. Your account needs an eligible Microsoft 365 subscription. Microsoft does not say whether this method works offline or processes the file only on your computer.
How do I open an MSG file on Mac?
Open Outlook on the web in a browser on your Mac and drag the MSG file into the reading pane. Your account needs an eligible Microsoft 365 subscription. Microsoft does not document a native Outlook for Mac route or Mac-specific compatibility limits in its MSG opening instructions.
How do I open an MSG file on a phone?
For reading on a phone, ask the sender for a PDF or HTML copy exported from classic Outlook. Keep the MSG if you need its original item structure. Microsoft does not document MSG support in mobile Outlook or a phone-specific browser procedure in its MSG opening instructions.
Why will my MSG file not open in new Outlook?
First check that the account has an eligible Microsoft 365 subscription and try the documented reading-pane opening method. Corruption is another possible cause: some MSG streams contain declared lengths that readers can compare with actual lengths to detect structural inconsistencies. An opening failure alone does not prove corruption.
Should I use MSG or HTML for saving an Outlook email?
Use MSG when you need the Outlook item's properties and attachment structure. Classic Outlook's HTML export is useful for viewing formatted email, images, links and selected header information. Keep the original MSG if you need the complete item; HTML is a reading copy.
How do I convert an MSG email to PDF?
For an email opened in classic Outlook on Windows, print it using Microsoft Print to PDF. The PDF is a viewing copy; do not assume it preserves the full MSG item's properties and attachment objects.
What does converting an MSG email to plain text lose?
Plain text loses bold, italics, font colors and the ability to display inline images. Keep the original MSG if you need the original message presentation and item structure.
How can I extract attachments from an MSG file?
MSG stores each attachment in a separate storage object. Open the item in a supported Outlook route and inspect its attachments. Microsoft's MSG opening instructions do not specify an extraction command or standalone extraction tool. A PDF or text export should not be assumed to preserve the attachment objects.
Are attachments in an MSG file safe to open?
Do not infer attachment safety from the MSG extension or successful opening. Microsoft identifies attachments as a common virus transmission route, and Outlook blocks some executable and other attachment types. Check the sender and the attachment before opening it.
Related formats
Sources
[MS-OXMSG]: Outlook Item (.msg) File Format · Microsoft 官方格式规范
[MS-OXMSG]: Top Level Structure · Microsoft 官方格式规范
Microsoft Outlook Item (MSG) · 美国国会图书馆
Open .eml, .msg, and .oft files in new Outlook and Outlook on the web · Microsoft 官方支持
Change the message format to HTML, Rich Text Format, or plain text in Outlook · Microsoft 官方支持
Save an Outlook message as a .eml file, a PDF file, or as a draft · Microsoft 官方支持
Blocked attachments in Outlook · Microsoft 官方支持