This problem most often turns out to be a typing error. Please double-check the accuracy of your recipient's e-mail address. However, if your recipient's e-mail address is correct, their internet service provider could be routing the ecard notification to a bulk folder by mistake. For example, Yahoo sets up bulk folders to intercept what they consider to be bulk mail or "spam" allowing only personal messages through to the inbox.
To assure that your ecards are received by your intended recipient, you may wish to send the ecard to yourself and then forward the notification that contains the ecard address to the recipient. You may also forward the copy you have received to the recipient.