Hi,
Before answering this question, can you please confirm the application server of Identity Suite. If it is wildfly8.x, then there is a possibility of configuring custom server configuration for smtp relay server as given below an example. The wildfly documentation is saying that we can add custom properties using property tag. But personally I did not try this as I don't have relay server to test.
<mail-session jndi-name="java:/Mail" name="IMMail">
<custom-server name="smtp" outbound-socket-binding-ref="mail-smtp">
<login name="username" password="password"/>
<property name="host" value="mail.example.com"/>
<property name="auth" value="false"/>
</custom-server>
</mail-session>