I have an SBS2003 server with Exchange setup using the POP and SMTP Connectors. Users are sending and receiving emails just fine except when sending to non-Exchange users with the same email domain. Example: firstname.lastname@example.org sends to email@example.com. I found this article on Microsoft that addresses this exact issue http://support.microsoft.com/kb/300681
. I am not updating users with Recipient Policies as this was causing a problem with changing the Reply To email address. I entered the ISP's mail server where the article tells me to as ISPMAILSERVER.COM and are no longer getting the error message and emails are gertting sent out from Exchange but are never received by the recipients. When I change the ISP's mail server to SMTP.ISPMAILSERVER.COM, I get the error message in the title "Recipient address rejected: Access denied". The ISP Mail server requires authentication on their outgoing email so I am assuming this is the cause but I don't know where in Exchange to enter login credentials to allow emails to get through.