Problem with mail server? rejected recipient address..?

Hi,
I have 2 bits of code to send an automatic email. One to myself and one to the address that the user places in a text box.
mail.myaddress.com is the right smtp address given to me by my host.

I get the error:
The server rejected one or more recipient addresses. The server response was: 550 <email address entered in textbox> No such user here
??? can anyone help? my code is: (this is in button click)
SmtpMail.SmtpServer = "mail.myaddress.com";
       
        string strrTo = "main@myaddress.com";
        string strrFrom = "noreply@myaddress.com";
        string strrSubject = "New Picture Uploaded& ...