Just my point of view, I take nothing given to me anonymously seriously. It has zero value to me. I look at answers here, harmful ones that thankfully most ignore, still they have nothing vested in giving you an answer of value.
Same thing for anonymous letters or emails. You don't believe in something enough to attach your name? If you are correct, if you are trying for the greater good, then you are not a trouble maker, why should you worry about attaching your name? Because you are a trouble maker and no one should take your emails as anything but nonconstructive criticism?
I get not wanting to confront but if your fear is being seen as a trouble maker, chances are you are being a trouble maker.
If I have a problem with my children's school, I don't email, I don't confront, I walk in, state my concern and ask how can I help. Sometimes after hearing their point of view I realize I was wrong, other times we work together to solve the problem. Seems a little more productive than a nonconstructive email.
Even a coward has no problem asking how they can help. I mean, you want to help right?