When the receiving server sends back a delivery failure notice, HawkOpen reads it from your connected mailbox and marks the address as a hard bounce (permanent, for example the address does not exist) or a soft bounce (temporary, for example a full mailbox). After a hard bounce HawkOpen stops sending to that address.
If nothing came back, check the address for typos, look for a delivery failure notice in your inbox, and ask the recipient to look in spam and to add you to their contacts. A domain without SPF, DKIM and DMARC is a frequent cause of silent filtering.