How to open a winmail.dat file
Open winmail.dat in your browser and get the trapped attachments back. What the file actually is, why you received one, and how the sender can stop it happening again.
A winmail.dat is not a corrupt file and not a virus. It is what arrives when the sender's Outlook packed the message into Microsoft's TNEF format and the mail system in between could not unpack it. The formatted message and — the part that actually costs you time — every real attachment are sealed inside it.
Why you received one
Outlook can send mail in Rich Text Format, a Microsoft-only flavour that standard internet mail has no way to express. When it does, Outlook bundles everything that does not fit — the formatting, and every attachment — into a single blob called TNEF, short for Transport Neutral Encapsulation Format. Another Outlook on the receiving end unpacks it silently and you never learn it happened.
Anything else — Gmail, Apple Mail, Thunderbird, a phone, a ticketing system, an archive — does not know how, so it hands you the raw blob under the name Outlook gave it: winmail.dat. Two things follow from that:
- It is not your fault and not the sender's fault. It is a setting on one message, or on one Outlook contact entry, that neither of you probably knows exists.
- Your attachments are not lost. They are inside the file. This is the important part, and it is the reason a winmail.dat is worth opening rather than replying to ask for a resend.
Open it here
- Save the
winmail.datout of the message. - Drop it on the viewer.
- Read the message, and download the attachments individually or as a ZIP.
Parsing runs in your browser. Nothing is uploaded, which matters more here than usual: a winmail.dat almost always turns up in a thread that was never meant to leave the company, and the obvious alternative — a converter site that wants you to upload it — means posting exactly that to a server you know nothing about.
What comes back, and what does not
The subject, the sender, the date, the formatted body and every attachment are all stored in the file and all come back.
The recipient list does not, and no tool can recover it from this file. TNEF is an attachment inside a carrying message, and To, Cc and the internet headers belong to that carrying message rather than to the attachment. If you need them, they are in the mail you received the winmail.dat with — open its headers there. A viewer that shows you a recipient list from a winmail.dat is guessing.
How the sender stops it happening again
Worth passing on, because it fixes the problem at source rather than one message at a time. In Outlook on Windows:
- For one message: in the compose window, open Format Text and choose HTML or Plain Text instead of Rich Text.
- For everything: File → Options → Mail, and under “Compose messages” set the format to HTML.
- For one stubborn contact: this is the usual culprit when it happens to the same person every time. Open the contact, double-click the email address, and set “Internet Format” to send plain text or HTML rather than Outlook Rich Text. A cached autocomplete entry can hold the old setting, so delete the address from the To field with the X in the dropdown before retyping it.
Related symptoms
The same message may also reach you as Part 1.2, ATT00001.dat or a nameless attachment — all of them are the same TNEF blob under a name some intermediate server invented, and all of them open the same way. If the file you have is an Outlook message rather than a TNEF blob, the won't-open troubleshooting guide covers that case, and what a .msg file is explains the format it is in.