This is caused by the symbol used for the quotation mark not existing in the standard UTF-8 text encoding system. This usually happens when:
1) The text for the email is copied from Word, or another editing program that is using a different font or text encoding setting
or 
2) The text for the email is entered into Sphere from a Mac, or a Windows system with a different text encoding setting

Below is an example of good and bad marks, and how to tell them apart. Note that the incorrect quotation marks appear more slanted.
Quotation Error
You can correct this by deleting the bad quotation marks, and re-typing them directly in the content/email editor within Sphere.