E-mails get marked as undeliverable due to one of two main reasons:
1. E-mail address of the recipient is not correct:
2. People often give invalid e-mail addresses intentionally or accidentally
E-mails are considered SPAM:
Emails can be considered SPAM depending on the content of the e-mail and the SPAM filter program being used by the recipient. You can change your content to try and make sure that words considered spam are not in the text of your e-mail. However ultimately the control of what is considered SPAM rests in the hands of the recipient. They often need to mark your e-mail address as part of their "white" list to ensure that your e-mail gets through.