Open a winmail.dat — and get the attachments out
Drop the winmail.dat from the email and see what Outlook packed into it: the attached files under their real names, ready to save, and the text of the message. It is read in your browser — the file never leaves your device.
How do I open a winmail.dat file?
You need something that reads TNEF, the format Outlook uses to carry a message's properties, because that is all a winmail.dat is: the attachments and text of one email, sealed by Outlook when it sent Rich Text mail to a program that does not read it. Outlook on Windows unpacks it, which does not help when the email went to Gmail, Apple Mail or a phone. Mailward opens it in the browser on any of them: drop the winmail.dat (it may be called ATT00001.dat) on this page, and every attached file is listed under its full name, ready to save byte for byte, with the message text beside it. Nothing is uploaded — the file is parsed on your device. Opening and reading are free, and so are the files of three messages a day; if winmail.dat keeps arriving, ask the sender to switch Outlook to HTML mail, which stops it at the source.
Save the winmail.dat (some mail programs call it ATT00001.dat) from the email, then drop it here — read locally.
See every attached file under its full name, and the message text beside the list.
Save the files you need: three messages a day are free, and every file inside one winmail.dat counts as one message.
Keep the whole email as one .eml or PDF, with the attachments inside it.
Open a winmail.dat — and get the attachments out
A winmail.dat turns up when someone sends from Outlook in Rich Text format and your mail program — Gmail, Apple Mail, Thunderbird, a phone — does not read Outlook's own format. The files they attached are sealed inside it, and nothing on your computer opens it.
Mailward reads the format, TNEF, directly in a Web Worker: every attachment is listed under its full name rather than an eight-character stub like REPORT~1.PDF, each one saves byte for byte, and the message text comes out of whichever body the sender's Outlook wrote. Pick several winmail.dat files at once and they open as one list.
Nothing is installed and nothing is uploaded.
What Outlook packs inside a winmail.dat
TNEF — the Transport Neutral Encapsulation Format — is how Outlook and Exchange carry a message's MAPI properties through ordinary internet mail. A winmail.dat begins with the bytes 78 9F 3E 22, followed by a run of attributes, each a level (message or attachment), an id, a length, its data and a 16-bit checksum. Some attributes are simple — the subject, the sent date, an attachment's bytes — and two kinds carry whole MAPI property lists, one for the message and one per attachment. That is where the useful parts sit: the attachment's full file name and MIME type, the body, the recipients.
Names are what a quick extractor gets wrong first. The attribute that labels an attachment holds its 8.3 short name — QUICK.DOC, REPORT~1.PDF — in the sender's DOS code page; the real name is the PR_ATTACH_LONG_FILENAME property inside the attachment's list, stored as Unicode. Mailward takes the long name whenever it exists, so a Chinese, Greek or accented file name arrives as written, and falls back to the short one only when nothing else was stored.
The body comes in one of three forms, and the best one present wins. Exchange Online writes the HTML as a property of its own. Older Outlook writes PR_RTF_COMPRESSED — LZFu-compressed Rich Text — and that is decompressed and read two ways: when Outlook encapsulated the original HTML inside the RTF, the HTML comes back character for character; otherwise the Rich Text is reduced to its text, paragraphs, line breaks and tabs, decoded through each font's code page so Chinese, Japanese and Cyrillic read correctly. An email attached to the email is a TNEF stream of its own inside an object property; it is saved as an .eml any mail program opens, with its attachments.
Checked against files nobody here wrote: Apache POI's test winmail.dat holds five attachments, and all five come out identical, byte for byte, to the originals POI ships beside it; its compressed RTF decompresses to exactly POI's message.rtf. Two Exchange Online samples give their subject, sender, recipient, HTML body, a 193 KB PDF and a 122 KB PNG whole. A file cut short by an unfinished download keeps every attachment that ended before the cut.
- Signature
- 78 9F 3E 22 — TNEF, 0x223E9F78 little-endian
- File names
- Full Unicode name first; the 8.3 title only as a fallback
- Body
- HTML property, plain text, or compressed RTF (HTML recovered when encapsulated)
- Attached emails
- Saved as .eml, with their own attachments
- Other names
- ATT00001.dat or no extension at all — recognised by the first bytes
- Tested on
- Apache POI's HMEF samples — 5 of 5 attachments byte-identical
- Free
- Open and read any number; save the files of three messages a day
Comparing formats? PST, OST, OLM, MBOX & EML explained — what each one is internally, and what is lost converting between them.
Before you trust a mail file to a website
Q. What is winmail.dat and why did I get one?
It is Outlook's packaging for a Rich Text email. When Outlook (or the Exchange server behind it) sends a message in Rich Text format to someone outside, it puts the formatting and the attachments into one file in its own format, TNEF, and attaches that as winmail.dat. Outlook on the receiving end unpacks it; Gmail, Apple Mail, Thunderbird and phone mail apps show it as a mystery attachment. The files the sender attached are inside — nothing is lost, it is just wrapped.
Q. How do I open winmail.dat on a Mac or an iPhone?
In the browser, on this page. Save the winmail.dat from Apple Mail (or tap it in the Files app on an iPhone or iPad), then drop or pick it here; Safari, Chrome and Firefox all work, and nothing needs installing. On a phone, the saved attachments land in Downloads, where Files can open or share them.
Q. Why are the file names inside different from what other tools show?
A winmail.dat stores two names for each attachment: an old DOS-style 8.3 title such as REPORT~1.PDF, and the real file name as a separate Unicode property. Tools that read only the first give you truncated, upper-case names and mangle accented or Chinese characters. Mailward uses the full name whenever the file carries one, and the short one only when the sender's client left nothing else.
Q. Is it safe to open a winmail.dat here?
The file is parsed in your browser tab as data — nothing in it runs, and it is never uploaded, which matters because the attachments are someone's documents. Saving an attachment writes its bytes exactly as the sender attached them; treat an unexpected .exe or macro document the way you would in any email.
Q. How do I stop receiving winmail.dat attachments?
The fix is on the sender's side. In Outlook for Windows: File → Options → Mail → Message format, set "When sending messages in Rich Text format to Internet recipients" to Convert to HTML format, and write new mail in HTML. For a whole organisation on Exchange Online, an administrator can stop TNEF going out to the internet with Set-RemoteDomain Default -TNEFEnabled $false. Until then, this page gets the files out.
Q. Can I open several winmail.dat files at once?
Yes, free: select them together and they open as one list, one message per file, searchable by subject, sender, text or attachment name. A file that is not a winmail.dat is skipped with the rest still opening. Saving files is the three-messages-a-day allowance; a Pro license lifts it and exports everything at once.
Free viewing, search, and 3 single-message exports a day. MBOX reading is free through the first 500 MB; listing and search cover the whole file. Batch conversion is a one-time $19 licence — no subscription, no recurring fees.
Other formats & archives
Same local-first engine, every common mail format.