The problem is caused by an incompatibility in the way that Outlook can send emails and the way that Apple Mail reads those messages.
In order to resolve the issue users need to change the outgoing email format and resend the message.
- On the sending machine, open Outlook.
- Click on File > Options > Mail
- In Compose messages, ensure HTML or Plain Text option is selected for the Compose messages in this format.
- Under Message format, set When sending messages in Rich text format to Internet recipients to Convert to HTML format or Plain Test format
- Click OK to finish
Resend the message