Share emails by Guests not being sent

When I am not logged in to Slideshare or others are not, the email to share a slideshow with a friend appears to be processed but never goes though to the recipient. If I am logged in the email reaches it's destination just fine. Why won't it go through when shared by a guest, even if they fill out the form correctly, including special characters, etc. This may not be a problem when sharing from within the player itself. But it never works when using the Share link on the webpage even though it looks like everything works and a confirmation message is provided.