There is a send limit on eCards and when that limit has been reached the error message is produced. A constituent can send 3 messages per hour and up to ten people on each message. So they'll be able to send to up to 30 people per hour from a specific IP address.
The limit is set to ensure that the application servers that process these emails are not overloaded. The eCard is still functioning but it may take some time before you are able to send from the same IP address again.