Can you send an email via Joomla mass mail component?
I suspect you may have entered an address as the system mail address that is not registered on the server and therefore it is stopping the mail being sent.
Some servers have a spam configuration set where you cannot send email from anywhere on the server if the senders mail address is not registered on the server.
Geraint wrote:can you email Joomla articles using the same mail icon?
Either there is a problem with the email address you are sending too (or from) or there is a problem with the Joomla com_mailto component.