winmail.dat Viewer for Outlook Attachments

Online tool for opening winmail.dat — the attachment that Outlook sends in place of a letter’s files when it formats the message in its own way. It shows the attachments under their real names, the letter text, meeting invitations and forwarded emails, and saves the files one at a time or all at once. It also works when the file is called ATT00001.dat or arrives inside a saved .eml letter.

How to use it

3 steps

Save winmail.dat from the letter

In the mail app, save the attachment called winmail.dat (sometimes ATT00001.dat) to your computer or phone. You can also save the whole letter as .eml.

Drop it here

The file is read in the browser: the list of attachments, the sender, the date and the letter text appear at once.

Take the files

Download any attachment, or all of them at once. Pictures, PDFs and text files can be previewed first.

Open winmail.dat and save the files that were sent with the letter

Drop a file here

winmail.dat, ATT00001.dat, .tnef — or the saved letter (.eml) with winmail.dat inside ·

Features

Real file names

Outlook keeps the full name of each file separately from a short 8.3 copy like CONTRA~1.DOC; the full one is used, in any alphabet the sender wrote it in — Cyrillic, Korean or German umlauts.

The letter, as it was sent

The text is shown as formatted HTML or as plain text. When Outlook kept it only as compressed RTF, the original HTML is restored from it, and an Outlook rich-text letter can be saved as an .rtf file for Word.

A letter inside the letter

A forwarded email arrives as a second winmail.dat packed inside the first. It opens in place, with its own text and its own attachments.

Meeting invitations to the calendar

An Outlook invitation that came as winmail.dat shows its time, place and organizer, and saves as an .ics file that Google Calendar, Apple Calendar and Thunderbird import.

Any file name, or none

A winmail.dat is recognized by its first bytes, so ATT00001.dat, “noname”, a .tnef file, a saved .eml letter or a base64 text dump of the part open just the same.

Large attachments without a wait

Only the structure of the file is read; an attachment is copied out at the moment you save it, so a winmail.dat carrying a large video opens as fast as a small one.

What is winmail.dat and why did I get it instead of the attachments?

It is how Outlook packs a letter in its own TNEF format (MIME type application/ms-tnef): text, formatting and every attached file in one part. Outlook sends it when the letter is written in “Rich Text” or when the sender’s mail server thinks the recipient also uses Outlook. Gmail, Apple Mail, Thunderbird and phone mail apps do not unpack it and show a single winmail.dat instead. The files are all inside it, byte for byte.

winmail.dat opened, but there are no files in it. Were they lost?

No. Outlook sends winmail.dat even when nothing was attached — then it carries only the letter’s formatting and text. If the sender meant to attach something and it is not in the list, it was not attached on their side; ask them to send it again.

How do I stop people sending me winmail.dat?

It is fixed on the sender’s side. In Outlook: File → Options → Mail → “Compose messages in this format” set to HTML, and under “Message format” choose “Convert to HTML format” when sending Outlook rich text to Internet recipients. A company administrator can switch TNEF off for outside domains on the Exchange server (Set-RemoteDomain -TNEFEnabled $false).

The file is called ATT00001.dat or has no extension. Will it open?

Yes. Mail apps rename the part freely — ATT00001.dat, noname, Untitled attachment — but a TNEF file always starts with the same signature, and that is what is checked here, not the name. A saved .eml letter with winmail.dat inside opens too.

How do I open winmail.dat on an iPhone?

In the Mail app, press and hold the winmail.dat attachment and choose “Save to Files”. Then tap the upload area on this page, choose “Browse” and pick the file. Each attachment is then saved back to Files, Photos or another app with the “Download” button.

I got a meeting invitation as winmail.dat. How do I add it to my calendar?

Open the file here: the invitation shows its time, place and organizer. The “Add to calendar (.ics)” button saves a calendar file that any calendar imports. Times are written in UTC, so the calendar shows them in your own time zone. For a repeating meeting the file holds its first occurrence. Accepting or declining still has to be done by replying to the organizer.

Is winmail.dat safe to open?

winmail.dat itself is a container and runs nothing. The files inside are whatever the sender attached, so treat them like any attachment: do not run programs, macros or scripts from a sender you do not know. The letter’s HTML is shown here with scripts removed and without loading pictures from the internet.

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Who is this tool for?

🩹 File recovery

Published 05 Nov 2026 Author: The Supreme