Recipients mail server is not responding
This error isn't as specific as hoped, however it does confirm the issue is not regarding your server (or outgoing mail server). The issue is specifically regarding the recipient's incoming mail server.
Knowing that the issue is on behalf of the receiving server, there is not much that can be done. You can attempt contacting the server administrations for the recipient's server, however using an alternative method for contacting the recipient would be the preferred solution.