We had a user not able to use the E-mail Users option on a SharePoint group. When selecting all the users to email she would get the error, "Could not perform this operation because the default mail client is not properly installed". Rather a misleading error, as her mail client was installed just fine.
So the fix? Select fewer people to email to. In this user's case, she'll be better off managing her email group in Outlook vs trying to use SharePoint.