being defensive is the perfect proof that you are guilty as charged，a big no-no, 在任何时候，一定要冷静。
真到了要借刀杀人，把你除掉的地步，你那样写的email也拯救不了自己的名誉。从你老板的角度，转发intern的email，不是对你的批评和指责。你老板的意图是什么：here we got a problem, the intern is stuck, since you worked on the data last year, maybe you could find a path forward, so the inexperienced intern could move on.
相对与那个水平不行的inetern, 你是水平很行的professional，所以你老板的意思是让你帮助、指点一下那个rookie，及时不能完全解决，至少让他有个方向继续做下去。
I just let you know at least one person's comment － 这不是轻描淡写，it's another way to say 'I am very disppointed to your defensive response'.
On top of taoqibao's excellent suggestions, I'd reply the email like:
Dear johny (the boss),
Thanks for forwarding the email. Based on my experience of working on the data last year, I have to say, I share Rookie's frustration that the data costs us lots of trouble. Due to the tight deadline, and I had couple of other projects on parallel, I didn't have the luxury of digging too much on the data.
However, I have been keeping thinking about improving the project since then. I am glad that I could offer my help to Rookie (the new guy), who got stuck in this project. I would love to share my thoughts with Rookie, and contribute design ideas. Shall we set up a meeting, so we can sit together and have further discussion ....blah-balh-blah....
这样写email，不动声色地把data责任交代清楚了，麻袋上绣花，底子不中，非我code之罪，而且用非常positive的态度, 为存在的问题提供possible solution, 让一个人在你的指导下干活，提升自己的地位，you would show team work, leadership etc... and another couple of sexy words in your portrait, you name it.
一句话来总结，就是前几天易老师说的：用positive的方式，解决negative的问题。
