In the specific environment, the mail server requires authentication from each entity utilizing that server, for sending emails.
Normally, in a non vApp environment, one would edit standalone-full.xml / standalone-full-ha.xml and add the following info - using the username / password details to the smtp-server section
<mail-session jndi-name="java:jboss/mail/Default" name="default" from="firstname.lastname@example.org" debug="true">
<smtp-server outbound-socket-binding-ref="mail-smtp" username="email@example.com" password="admintest"/>
However, in vApp we are unable to take modify this parameter as not having access to modify this parameter.
It would be useful adding another custom property for allowing authentication to take place.