It actually sounds like your manager's response was useless and left you cold. Regardless of any action you take, it is important to recognize at the outset that a person who writes this type of letter is not typically inclined to listen to constructive feedback.
Even if your idea was ridiculous, there is no excuse for her response. All that her behavior has done is generate anger and squelch the likelihood of your ever presenting another creative idea to her. As a result, she loses, the company loses, and, unfortunately, you lose.
Since you are so upset over her reaction, you should meet with her. This should not be a meeting where you evaluate her or the letter, but rather one in which you try to clarify the ground rules. Tell her that you were confused by her response to your idea, and you would like to know if there is anything you should be doing in order to avoid this type of reaction in the future. Her reply will tell you if it is a good idea to continue in her department, or if it is a better idea to broaden your horizons.
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