My company uses a lot of em dashes in their copy and it seems like they break all the time. Specifically when emails are being forwarded from a Microsoft Exchange server. I feel its all about the type of encoding that the exchange server is using but thought I would ask anyways.

I have this in the header of my emails:
<meta charset="utf-8" />

I also have tried these character encoding options: &#8212; and &mdash;1