asp.net 2.0 - send email

Hi,
This is the error that I am getting when attempting to send an email via asp.net 2.0.
Mailbox unavailable. The server response was: 5.7.1 Unable to relay for [email protected]

Any ideas please?
THanks

The server which you are attempting to use to send an email is not allowing you to send emails TO it or THROUGH it. It is a server configuration issue. Show code?

Hi,
I have now managed to partly fix the issue.
When sending email to an address outside the office then it works fine, but when sending to an email inside the office no error is given but the other address does not receive the email.
Thanks