"Prism is unable to communicate with your email program."

Two related messages:

• "Either there is no default email client or the current email client cannot fulfill the messaging request"
• "Prism is unable to communicate with your email program."

A common reason this message comes up is that Prism is a 32-bit program, and can't communicate with a 64-bit version of Outlook (and maybe some other email programs). 

The workaround, of course, is to save your Prism file, create your email and attach the file.