I'm currently able to send emails using the typical C# SMTP through Exchange 2007 (as long as the from / to are within my domain). But when I try to send them outside the domain, I get this error message:
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?