Hi John,
Based on my research, this is a known issue between Outlook for Mac and Outlook for Windows. The easiest workaround would be to send e-mails as plain text.
Since this forum focus more on questions and feedback related to Outlook for Windows, if you need further assistance regarding this issue, I'd recommend you post a new question in the Outlook for Mac forum:
http://answers.microsoft.com/en-us/mac/forum/macoutlook?tab=Threads
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Steve Fan
TechNet Community Support