Regardless of company size, it is crucial to remember that you exist to serve your customers.
In startups, team members might think it is understood that customers should be treated with respect; unfortunately, this is not always common sense. Those who are not directly involved in sales & marketing might be careless when responding to customer emails. These individuals excuse their behavior by stating that customer support is not their specialty. However, friendly and polite communication should be explicitly required across the board, especially in small startup teams where the roles might be fluid.