Apple mail replying with a different (very wrong) SMTP server

Two separate businesses and email addresses. I can carefully chose the correct sever domain to start the email but when I reply apple mail chooses where to send it from. This is a very big problem when email comes from an entirely different business. Ive had customers complain about my email situation. I am Getting discouraged with apple never thought Id say that. (: Anyone share this issue or know a solution?