Guide
How to stop Outlook sending winmail.dat
If the people you email get a winmail.dat file instead of your attachments, your Outlook is sending in Rich Text format. One setting fixes it.
Updated 2 November 2026 · 2 min read
Why it happens
Outlook's Rich Text format packs the attachments into a file called winmail.dat. Outlook unpacks it again; Apple Mail, iPhone, Gmail and most other apps don't, so your recipients see winmail.dat instead of your files.
Change Outlook's sending format
- In Outlook for Windows, choose File, then Options, then Mail.
- Under Compose messages, set Compose messages in this format to HTML.
- Under Message format, set When sending messages in Rich Text format to Internet recipients to Convert to HTML format.
- Click OK. New emails go out without winmail.dat.
If one person still gets it
Outlook can remember Rich Text for one address. Start typing the address in a new email, select it in the suggestions that appear, and press Delete. Then type the address again in full.
For Exchange admins
You can stop winmail.dat for everyone sending outside your organisation. In Exchange Online PowerShell, run Set-RemoteDomain Default -TNEFEnabled $false.
Got a winmail.dat already?
The files are still inside it. The winmail.dat opener gets them out in your browser, so we never see them.