Message delivery to the host '203.97.33.212' failed while delivering to the remote domain 'clear.net.nz' for the following reason: An SMTP protocol error occurred.
The SMTP verb which caused the error is 'MAIL'. The response from the remote server is '451 #4.1.8 Domain of sender address <forums@geekzone.co.nz> does not resolve'.
Also
Message delivery to the host '203.97.33.212' failed while delivering to the remote domain 'paradise.net.nz' for the following reason: An SMTP protocol error occurred.
The SMTP verb which caused the error is 'MAIL'. The response from the remote server is '451 #4.1.8 Domain of sender address <forums@geekzone.co.nz> does not resolve'.
451 seems to be a temporary local problem, but the error message from your server seems to indicate otherwise.