A refusal by an ISP or mail server not to forward your email message to the recipient. Many ISPs block email from IP addresses or domains that have been reported to send spam or viruses or have content that violates email policy or spam filters.

Building a Highly-targeted Opt in List: When Money Lies Beneath the Lists
Are you successful with your Internet business? Better yet, do you think you are successful with your online business? When most online entrepreneurs are asked with these questions, they often brush it off.