Hi
Jethro,
Please try sending a email from Excel to yourself and see if it works. Also try changing the email format to Rich or Plain Text and test. (Other than HTML)
Click File, Account Settings, Account Settings, and click Data Files. If the PST file is set as the default change it to the email account. It would be the same default account you see on the E-mail tab in the Account Settings.
Once you make this change see if the problem still occurs. You might be coming across a Office 2013 bug.
Check if this article helps:
Email Body Text Missing When Sharing From excel 2013
Also would you mind listing out the steps used to do so, along with the Excel version you are using.