if the email server is placed internally and you are supposed to send email to internal users only than there is no need to allow the relay.
just put ip address of the messaging server and port 25 in the SMTP settings of your application setup.
also make sure to check your SMTP server's setting, if it is configured to "accept the message from Authenticated Users only" than you have to use username and password to send email from your application.
If you want to send email to external users as well than you have to Allow the relay using the SMTP configuration of your messaging server
