It is easy to forget, for example, to set Reply-To according to Mailer.DescriptorImpl.getReplyToAddress when sending mail from a Jenkins plugin, which can break acceptance-test-harness usage. There ought to be an easy API in mailer-plugin for generating a preconfigured MimeMessage. createSession is a good start but there should be a higher-level convenience function that presets the From, X-Instance-Identity, Date, and Reply-To headers and returns a MimeMessage ready for a subject and body and recipients and then Transport.send.