She sounds like she has had issues with you for a long time. Issues of jealousy and resentment. Its what I have discovered when an attack comes out of no where and when it seems over the top. My question is why did you boss get involved in the first place and why is email being allowed to be used as a form of harassment. I'd be hitting human resources if it were me. Let it go its only a work place meal. Ignore her she has bigger issues than you.