A “hard bounce” is the phrase used to describe the response an SMTP server sends to the originating transactional email server to indicate that the recipient email address does not exist.
Upon receiving a “hard bounce”, TitleLeaf marks the address as “inactive”. Further requests to send email messages to that address will be skipped. TitleLeaf does this to save bandwidth and ensure reputational integrity of the originating address (ie, further deliveries risk being reported as spam, which can lead to being blocked by the ISP).