That sounds like a tough letter to receive. Once you move past the hurt this letter caused, I suggest that you try to view the letter as someone trying to help you instead of hurt you though. Either you are perceived as yelling a lot by neighbors passing by or by children talking to their step-mom about their perspective and that is something you may want to explore more and try to problem solve.
It may be a simple matter of your children interpreting your emotions incorrectly and labeling the slightest irritations as yelling also, which is something you should explore if you think the letter came from their step-mom.
I am entertaining the possibility that this letter is truthfully from someone who walked past my house and heard yelling. I am more careful to keep my volume down. Thanks. My children are NOT the people who think I'm yelling. I don't know where you came up with that. They dislike their step-mom and beg me to just live at my house. I even asked them if they think I yell too much and they said no. I told them if they ever think I'm yelling, they can ask me to use my inside voice and they thought that was pretty funny.