The definition of a hard bounce is an email that could not be delivered because of permanent problems. A typical reason is that the email address does not exist ("mailbox not found"). This often happens when either the contact name or the domain name has been misspelled (e.g. the email address w.wilson@example.com is spelled as w.wilson@exmaple.com or w.wislon@example.com).
To resolve this scenario, the best solution is to verify the acquisition processes and use either an email validation tool or/and have a DOI process in place.
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