Refactoring of envelope from setting, to fit with other code updates and to enable setting via sendMail closure:
sendMail {
    envelopeFrom '...'
}

See http://www.pither.com/articles/2010/06/22/envelope-senders-for-grails-mail-plugin-spring-java-mail
Add simple unit test for envelopeFrom
Object to using envelopeFrom without JavaMailSender
Integration tests to cover envelopeFrom use
Tabs not spaces for consistency.
Stronger typing of GreenMail