TNEF explained
What Is a Winmail.dat (TNEF) File?
Winmail.dat is what Outlook sends when it packages a message and its attachments into Microsoft’s TNEF format. Other mail apps show the container instead of the files inside — the originals are usually still recoverable.
What TNEF means
TNEF stands for Transport Neutral Encapsulation Format. Instead of sending each attachment as its own MIME part, Outlook can add one application/ms-tnef part holding the message formatting, properties, and files. Mail apps that cannot decode TNEF display that part as winmail.dat.
Microsoft documents the format in [MS-OXTNEF]: Transport Neutral Encapsulation Format (TNEF) Data Algorithm.
Why Outlook creates it
TNEF is used when Outlook sends in Rich Text Format, when a message uses Outlook-only features such as voting buttons or meeting requests, and when a contact or an Exchange server is configured to send TNEF to external recipients.
What’s inside
A winmail.dat container can hold the message body and its formatting, file attachments, meeting invitations, and contact cards. Attachments keep their original names and types, so a recovered PDF is still a PDF.
What problems it causes
Apple Mail, Gmail, and most mobile apps cannot decode TNEF, so the message arrives with no visible attachments and a single winmail.dat file. Nothing is lost — it just needs a TNEF-aware opener.
Missing Outlook attachment? →How to open it
Read the message free in the browser viewer, or open the original file in Winmail File Viewer for iPhone and iPad to view and save the attachments. Follow the five-step guide.
How to prevent it
Ask the sender to use HTML or plain text instead of Outlook Rich Text. Administrators can change TNEF settings for external recipients.
Frequently asked questions
Are winmail.dat files dangerous?
The container itself is not malicious, but treat the files inside like any email attachment and only open ones from senders you trust.
Why does only Outlook create them?
TNEF is a Microsoft format used by Outlook and Exchange; other mail clients send standard MIME attachments.
Will winmail.dat files go away?
They appear whenever a sender or server uses Outlook Rich Text or TNEF, so the setting has to change at the source.