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