What is winmail.dat?

winmail.dat is a TNEF (Transport Neutral Encapsulation Format) file that Microsoft Outlook or Exchange creates when it sends a message using Outlook’s proprietary “Rich Text” formatting to a recipient whose mail server doesn’t understand that format. Instead of a normal message with attachments, you receive one empty-looking email with a single winmail.dat file attached — which actually contains the real message body, formatting and attachments bundled together.

Why it happens

It comes down to one setting on the sender’s side: Outlook can format outgoing mail as HTML, plain text, or Rich Text Format (Outlook-specific). Rich Text is the only one of the three that non-Outlook (or non-Exchange) recipients can’t read directly, so their mail server — or their own Outlook, if it can’t reach your Exchange server — packages everything into winmail.dat as a fallback.

Fixing it at the source (best option)

If you send winmail.dat files to other people, the real fix is on your end: in Outlook, set the message format to HTML or Plain Text instead of Rich Text, either for that one contact (Outlook lets you set format per contact) or as your default. This avoids the problem entirely for every future email.

Opening one you’ve already received

If you’re on the receiving end of a winmail.dat file:

• A browser-based TNEF/winmail.dat opener extracts the real message body and attachments locally, without needing Outlook.
• Some webmail providers extract it automatically and show the attachments as normal — try opening the original email in Gmail or a similar provider if you have that option.
• Asking the sender to resend as HTML or Plain Text (see above) avoids needing a workaround at all.

What’s inside

A typical winmail.dat contains the formatted message body plus any real file attachments the sender included — none of it is lost, it’s just wrapped in a format your mail client didn’t expect.