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Winmail.dat Opener & Viewer

Open a winmail.dat file in the browser and pull the real attachments out. Parsing is local TNEF — nothing is uploaded. A winmail.dat file is not an email you can double-click; it is the wrapper Outlook uses when it sends rich text to someone who is not on Outlook.

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Written by Casey Marlin · Last updated
This update: TNEF attribute stream and compressed-RTF handling checked against MS-OXTNEF / MS-OXRTFCP and synthetic fixtures

Drop a winmail.dat file here

Or click to pick one. Parsed in this tab — nothing is uploaded.

What is a winmail.dat file?

A winmail.dat file is Microsoft's Transport Neutral Encapsulation Format — TNEF. When Outlook composes a message as Rich Text, it can pack the formatted body, the true attachments, and a handful of Outlook-only properties into one binary blob and attach that blob as winmail.dat. Gmail, Apple Mail, Thunderbird and most phones do not speak TNEF, so the recipient sees one opaque file instead of the invoice.pdf that was actually sent.

It is not the same thing as an .eml or a .msg. An .eml is MIME text; a .msg is Outlook's OLE container for a whole message. winmail.dat is the leftover packaging on a MIME message that already arrived. If you have the original .eml, open it in the EML viewer first — the winmail.dat often sits in that message's attachment list.

How to open winmail.dat

  1. Drop the file on the opener above, or use the file picker. Names vary — winmail.dat, Winmail.dat, sometimes att0001.dat.
  2. Read the attachment table. Long filenames are preferred over the old 8.3 DOS names Outlook still stores. Download the files you need, one at a time, or use Save all.
  3. On a Mac or iPhone, skip the native Mail apps for this file; they will not unpack TNEF. Use this page in Safari. On Windows without Outlook, use this page in any browser.

If an unpacked file is itself an .eml, download it and drop it on the EML viewer. Nested messages are common in legal holds and helpdesk exports.

How to stop Outlook from sending winmail.dat

Fix it at the sender. In Outlook: File → Options → Mail → Compose messages in this format → HTML (or Plain Text). Rich Text is the setting that produces TNEF for internet recipients. For one correspondent, open their contact card, find the Internet format option, and stop forcing RTF.

Administrators can set the same policy in Exchange so the whole organisation stops wrapping outbound internet mail. Until the sender changes that, every non-Outlook inbox will keep getting a winmail.dat file instead of the attachments.

FAQ

Is my winmail.dat uploaded?
No. The TNEF stream is parsed by JavaScript in this tab. After the page has loaded you can disconnect the network and the opener still works. Attachments are rebuilt as local downloads. There is no conversion server and no account.
Why did someone send me a winmail.dat?
Outlook wrapped a rich-text message in TNEF. Recipients who are not using Outlook see one file named winmail.dat instead of the real attachments. The sender usually did not choose that filename; Outlook did it because the message format was set to Rich Text.
Can I open winmail.dat on a Mac or iPhone?
Yes, in the browser. Apple Mail and iOS Mail do not unpack TNEF. Open this page in Safari, pick the file from Downloads or Files, and download the attachments that come out. The same page works in Chrome on Android and on Windows without Outlook.
Is winmail.dat a virus?
The container itself is not malware. It is Outlook's packaging. The files inside it are ordinary attachments and should be treated as such: do not run an unexpected .exe or enable macros in a spreadsheet from a sender you do not trust. This opener lists names and sizes before you download anything; it does not scan for viruses.
The attachment list is empty — why?
Some winmail.dat files only store formatting (fonts, RTF markup) and carry no separate files. The body may still be recoverable as a best-effort text preview or an .rtf download. If the original email also arrived as .eml, open that in the EML viewer — the attachments sometimes live there instead of inside TNEF.
Can I read the email body too?
When Outlook packed compressed RTF into the TNEF stream, this page tries to decompress it and show a plain-text preview. That preview is best-effort: tables and embedded pictures will not look like Outlook. You can download the .rtf and open it in a word processor for a closer rendering.
How do I stop Outlook from sending winmail.dat?
On the sending side, switch the message format from Rich Text to HTML or Plain Text (File → Options → Mail → Compose messages in this format). For one contact, open their card in Outlook and set Internet format to “Let Outlook decide” or “Send Plain Text”. After that, non-Outlook inboxes receive normal MIME attachments instead of a TNEF blob.

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