Mostly such issues are about spam filters on the receiving end. BCC mail with not recipient, many listed in the to or CC or just the content of your message. All of those things can contribute to mail being considered spam. Then their are the other less scientific methods. Comcast used to (I do not know what they do now) black list emails from a address or server is two of their customers clicked the spam button on their web mail interface. Basically deliver-ability of list mail has become an industry in itself with large businesses operating in the space. For example mailchimp. http://mailchimp.com/