Over at the various Apple forums this is a common issue, you cannot send emails because the server does not allow relaying. There also seems to be no answers forthcoming from Apple while its users struggle to resolve the smtp issues.
Apple products such as ipad and iphone have caching issues and every now and again need to be turned off and restarted. In every case we have encountered of the “error relay message” this resolved the issue and the emails sitting in the outbox flew out at restart.
To properly turn off and restart an iphone or ipad hold the start button and the close switch at the top of the device together. Wait for the turn off your iphone message to appear, then continue to hold wait another second or two, the screen will go black. Then press the top start button to see the apple logo appear.
This should be done once a week in all Unix based systems and less frequently in Windows systems.