The default html.jelly template uses ${rooturl}static/e59dfe28/images/32x32/blue.gif. For some reason, this location requires authentication, hence the email is unable to load the email. Noticed that ${rooturl}static/e59dfe28/images/32x32/green.gif does not seem to have this problem (though it uses a different shade of green than the greenball plugin sets for blue.gif ... not sure if this was intentional or not).

This issue was found on Jenkins version 1.553 with the latestest released versions of the greenball and ActiveDircetory plugins.