Is it a customer? If so, ask yourself, how old is the customer? How long have they been with your company? Are they familiar with your products and services, or will they need extra hand-holding? When writing an email to a customer, it’s important to explain products, services, and terms and conditions in detail. How formal you are will depend on your established brand, but in general it is best to remain formal and friendly with customers. However, you don’t want to be too formal, because stuffy business language can confuse a customer and create more communication barriers. Use short sentences with no more than two clauses, and beware of using passive tense.