Please Help.

I have a new zoho business account (hobartcityfarm.com). 

Of recent when i've been trying to send emails from hello@hobartcityfarm.com or james@hobartcityfarm.com i've been getting an error: 

Sending the message content to the server failed.

The server response was: Relaying disallowed as Hobart city farm <hello@hobartcityfarm.com>

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.

Any help would be muchly appreciated

Cheers

James