Have progressed a little - this error behaviour is really helpful.
I am now getting the following error ??
mail() [function.mail]: SMTP server response: 550 <email@example.com> Relaying denied
Sending To: firstname.lastname@example.org... mail() [function.mail]: SMTP server response: 550 <email@example.com> Relaying denied - Failure
PS: Just changed domain name in the above to mydomain for security reasons.