What is Hard bounce?

The receiving server says no and means it. The mailbox was closed, the domain is gone, or the address was never real. No retry will work, because there is nothing on the other end.

Typos in a signup form cause a lot of them. So do old lists, and so does anything you bought instead of collected. A batch of hard bounces right after an import is the import telling you something.

Sending repeatedly to dead addresses is one of the clearest signals that a sender is careless. Most email platforms suppress hard bounces on your behalf. Confirm that yours does, and never re-import an address after it hard bounces.