I am able to send emails using the typical C# SMTP code across Exchange 2007 as long as both the from and to addresses are within my domain.
As soon as I try to send emails outside the domain I get:
Exception Details: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Unable to relay
How can I get exchange to accept my email and send it out to the internet?