This is an email I received from Dropbox.com:
I feel a sense of loyalty towards Dropbox because it’s a service I use, depend on and appreciate. But that positive feeling isn’t enough to move me above their free service. However, it does leave an imbalance of indebtedness on my part. This is where their secondary request (refer a friend) comes in. People who aren’t willing to upgrade are more willing to refer a friend.
Do your emails offer recipients secondary options?