Getting rid of forsaken “winmail.dat” sent by Microsoft Outlook

In many situations or scenarios Microsoft Outlook, instead of regular (consumable by other mail clients) meeting invitations can start sending “windmail.dat” instead. This is a proprietary format that other mail clients (like Thunderbird, Mac mail clients, web clients) ignore as not readable.

List of situations and configuration switches that may cause Outlook to behave like this is long. And so is the solution list. In my case only the most aggressive solution worked.

I’ve read through entire slipstick.com’s article reading through every chapter, checking every detail, all for nothing. Even after applying every suggested solution, receivers of my meeting invitations were still getting forsaken winmail.dat instead of regular invitation which they could accept or reject.

Finally, the solution was to block sending rich messages at Windows Registry level.

To follow this solution you need to:

1. Close Outlook
2. Hit Win + R type regedit and hit Enter.
3. Locate the HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences registry key (for Outlook 2013; for other versions — refer to original article).
4. Right click main Registry Editor’s window and select New > DWORD item from context menu.
5. Name it DisableTNEF and hit Enter to confirm.
6. Double click it and in the Value data box type 1.
7. Close the Registry Editor and restart Outlook.

This should solve your problem in most cases.

If it is still not solved (i.e. some of your contacts are still getting winmail.dat) you must clear whole auto-complete cache, by closing Outlook again and restarting it with:

outlook.exe /cleanautocompletecache

or by going to File > Options > Mail > Send Messages section and by clicking the Empty Auto-Complete Lists button.

This will unfortunately — as the name says — clear all your contacts auto-complete cache forcing you to start typing email addresses by hand or using contacts list again.