There are numerous suggestions and tools for reading WINDMAIL.DAT files.
Prevention is better than cure, right? So if you are the culprit and people are moaning that they are receiving these files from you, here is a permanent answer to prevent Outlook from formatting mail as WINMAIL.DAT

The problem occurs when you use Outlook and “Rich Text Format” to compose your emails.
Your customers and friends who try to read the email on Apple Mac devices such as iPad, iPhones and Macs using Apple Mail will find them unreadable. Outlook reformats your mail and attachments and bundles them into a “WINDMAIL.DAT” file.

Below is a quick guide to save you time and frustration and stop this from happening:
Applies to all versions of Outlook: Outlook 2003, 2007, 2010, 2013 and 2016. Mac versions: Outlook 2011 and Office 365 variations.

Instructions below are for Outlook 2016:

1. Open your Outlook email client.
2. Click on “File” located in the upper toolbar.

3. Click on “Options”

4.  Select “Mail” on the left hand tool bar

• under “Compose messages”, make sure  HTML (recommended) or Plain Text is selected in Compose message in this format:

5. Scroll all the way down, under “Message format”,

• set “When sending messages in Rich text format to Internet recipients:” “Convert to HTML format” (recommended)  or “Plain Text format”.

6. Click OK to finish.

Smiling faces again and no more arguing or calling TECH support in despair 🙂