Take a look at usermodule.java in core/user.
It creates a mail using MBean properties to configure.
Avoid deployment descriptor modifications (XXX-service.xml) as MBean properties has precedence. Use jmx-console instead.
You'll have to complete mail module properties : mode (most important), user, password, gateway if needed, validatefrom, validatesubject
Exemple with use of plain mail account : mode=1, smtpuser=username, smtppassword=password
See usermodule code, it's self-explanatory.
For attachments, I think it's not provided by mail module.