A hard bounce says 'an email you sent has returned because there is a permanent reason about the reachability of this subscriber'. Bounced email addresses need to be cleaned in your email list immediately either manually or automatically. Not excluding hard bounce email addresses will lead being flagged as spam in your future campaign sends. Also, repeated hard bounces could affect a sender’s reputation badly.
The common reasons why an email may hard bounce are listed below;
Recipient email address may not exist,
Domain name of sender doesn't subsist,