An email that is rejected by an email server for a permanent reason. An email may hard bounce if a recipient email address or domain name doesn’t exist—or the recipient email server has completely blocked the delivery. There are, however, occasionally times when a valid email address will hard bounce.

Combining Email Marketing with Other Types of Marketing
Whether you choose to do your marketing online or offline, one thing does not change. This is the concept that no single marketing strategy will be as effective as a combination of two or more marketing strategies.