My attachments sent to Mac users from Outlook turn into winmail.dat files. Why can't they open them?

This is due to Outlook decrypting the file in RTF format. To avoid this issue you may change your Outlook settings by following these instructions:

  1. Open up your Outlook email client.
  2. Click on File located in the upper toolbar.
  3. Select the Options setting.
  4. From here, select the Mail category.
  5. Now, under Compose messages, you want to ensure the HTML or Plain Text option is selected for the Compose messages in this format.
  6. Next, under Message format, set When sending messages in Rich text format to Internet recipients to Convert to HTML format or Plain Test format
  7. Click OK to finish