When sending emails from hudson, my mail server (postfix, but I'm sure others
will behave similarly too) rejects the message because the smtp HELO message
isn't being sent.

This is because the system property mail.smtp.localhost hasn't been set before
sending the email. This is described in the javadocs for com.sun.mail.smtp.

See also: http://forum.java.sun.com/thread.jspa?threadID=482673&messageID=2252508.

The solution is to add some code in hudson.tasks.Mailer.createSession() like:

props.put("mail.smtp.localhost", localServerName);

I guess some code to pull the localServerName out of config would be needed to.

In theory, a work around might be setting a system property when running hudson,
but I've not been able to make this work yet.