If there is no connection attempt logged by Exchange, it isn't Exchange denying access. You haven't made it to Exchange yet. Make sure the SMTP server listed in DNN is valid. It must be a FQDN that points to the Exchange server, such as exchange.sample.com. Or it must be the IP address of the Exchange server, in brackets, as in [192.168.1.10]. Exchange must be configured to allow relay from your server, or better yet all who authenticate. If you use authentication, you must provide a username and password that is valid on Exchange.
There must be no firewalls or other security software blocking port 25 between you and the Exchange server.
Jeff