Winmail.dat is a file that contains information about the message (and its attachments) witch is created using e-mail program Microsoft Outlook and it’s TNEF standards. This type of files is not supported by most of e-mail servers, and that's why, they can not be opened.
To resolve this issue, you must change the settings in your Microsoft Outlook program.
Microsoft Outlook 2007 and older :
1. Open the menu Tools -> Options -> Mail Format
2. In section “Message Format” press on “Internet Options” and choose “Convert to HTML format”
3. Confirm this activity by pressing the OK button
Microsoft Outlook 2010, 2013, 2016 :
1. Open the menu File -> Options -> Mail
2. In first section "Compose messages" -> Compose messages in this format select HTML
3. At the bottom of this page, in section “Message Format” press on “Internet Options” and choose “Convert to HTML format”
4. Confirm this activity by pressing the OK button
!! When settings is changed, you need to clear the autocomplete cache. If Auto-complete cache is not cleared, mail program continues to send messages through Rich Text Format, which ultimately becomes a winmail.dat file.
To clear the autocomplete cache :
1. Come into Microsoft Outlook menu File -> Options
2. In section “Send messages” press on the button 
3. Confirm this activity by pressing the OK
4. Logout and login in mail program again.
!! If the above actions have not helped, follow the instructions below.
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry.
To prohibit Outlook 2007 from sending the Winmail.dat file :
- Install hotfix 957692.
- Add the DisableTNEF registry entry - Exit Outlook 2007.
- Start Registry Editor.
- In Windows Vista, click Start
, type regedit in the Start Search box, and then press Enter.
If you are prompted for an administrator password or for confirmation, type the password, or provide confirmation. In Windows XP, click Start, click Run, type regedit, and then click OK.
- Locate and then select the following registry subkeys:
6. On the Edit menu, point to New, and then click DWORD Value.
7. Type DisableTNEF, and then press Enter.
8. Right-click DisableTNEF, and then click Modify.
9. In the Value data box, type 1, and then click OK
10. Exit Registry Editor.
11. Restart the computer.
To prohibit Outlook 2010 from sending the Winmail.dat file :
- Add the DisableTNEF registry entry. Exit Outlook 2010
- Start Registry Editor.
In Windows 7, click Start, type regedit in the Search programs and filesbox, and then press Enter. In Windows Vista, click Start, type regedit in the Start Search box, and then press Enter. In Windows XP, click Start, click Run, type regedit, and then click OK. If you are prompted for an administrator password or for confirmation, type the password, or provide confirmation.
- Locate and then select the following registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
-
On the Edit menu, point to New, and then click DWORD Value
-
Type DisableTNEF, and then press Enter.
- Right-click DisableTNEF, and then click Modify.
- In the Value data box, type 1, and then click OK.
- Exit Registry Editor.
- Restart the computer.
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