Writing feedback letter after leaving job.

Is it appropriate or helpful to send a feedback letter after I have left a company. Over the last few months my boss had become increasingly misogynistic and hostile toward me in particular (and a little toward other women) I see a very clear path from the day we announced ladies in our workspace taking the lead in a "celebrating women" event. His ego was bruised and myself and my coworkers were enemy number one since we were the ladies who work directly below him and "took over his thing" we had an arguement, I tried to hace a constructive conversation with him to no avail, I spoke to another manager for feedback on my perception of the situation. When the treatment worsened I went to HR, a month passed with no further action and increasing hostility. Friday I insisted on a meeting after being berated in front of all of my coworkers for asking a simple question. This was the last straw, I wrote my resignation letter before the meeting, but was hopefully he would see reason and soften his heart. When he didn't, I took that as my sign and I resigned.

I want to write a letter to the president of the company outlining the timeliness and giving them feedback on this man. For the sake of my coworkers, he needs to change. Is this unprofessional behavior? Is this something that is typically done? Will this just be perceived as sour grapes?