No Such User Error on ASP Form

I have a form on my site which allows my users to leave feedback messages.
When I test the form using my own address in the txtEmail text box on my form, it works flawlessly, however if an end-user completes the form using an address from another domain they get the error "Mailbox Unavailable. No User Here".
The cause of this error is obvious - The server is trying to stop me relaying by using an unknown mailbox / email address however if you look at my code as follows:
Dim WebformCustomer As New MailMessage() WebformCustomer.From = New MailAddress("noreply@techtosolutions.com", "Tech to Solutions") WebformCustomer.To.Add(New MailAddress(txtEmail.Text, strCustomerName))
As you can see from my code, I am using a valid address in my domain as the From address and the To address is the user entered variable.
Why would the server be requiring me to use a valid mailbox ...