We had the same problem, and though I'm not saying the answer isn't in the knowledge base, I didn't find it either.
We only ever rarely sent them out (reserved the disruption to our affiliates/members for big announcements... like "Hey, Guys... thought you might find it handy to know that we're changing our program name and url."
)... so I just resorted to abstaining from use of apostrophes and the like, and simply entered plain text, and all seemed to work well.
I'm sure there's gotta be a better answer, though. File a ticket, if nothing else - in my experience, John's peeps are real solid about their tech support when needed.