Is there a way to prevent that people who dont want e-mails get it by mistake?
Yes, there is a built-in function that allows you to permanent or temporary prevent contacts from receiving e-mails from you. Via the File – Exclusion List menu item you can add those e-mail addresses that you want to exclude from further mailings. E-mail addresses that are in an exclusion list will not receive e-mails during a mail delivery.