The most overlooked, easy, and successful proofing method is to just read your email back to yourself and see how it sounds. If you just type your email out and press send you will most likely miss simple context, spelling and grammatical errors.
Additionally, when reading back to yourself, don’t just scan the document for errors, read it out to yourself the way you would if you were reading an email you had received and had not yet read.
Make sure that you also read out the initial email or enquiry that you are replying to and ask yourself the following; Am I addressing what they were asking/saying? Have I covered all points they were asking about?