We have an application server that uses our Exchange infrastructure to relay emails to our internal users. Every thing worked great in Exchange 2003, but since upgrading to Exchange 2010, we've experienced an unexpected issue. The emails sent from the application server have multiple recipients. If all recipients are valid, Exchange with relay the email message. However, if one of the recipients is invalid, the email will be not be relayed to any recipient. Interstingly, the message sent back to the application server indicates that the exchange server accepted the message. I have searched my transport logs and cannot find the message. We didn't start having this issue till we upgraded to Exchange 2010 so I'm hoping it's just some switch I need to make on Exchange 2010.