HawkOpen recognises a reply when it can read your mailbox and the reply belongs to the same conversation; this works for IMAP/SMTP mailboxes and for Gmail or Outlook connected by signing in. With webhooks on the Business plan, a due reminder can also be sent to another tool.
An unopened email does not mean it was not read, and an open does not prove it was. When the matter is important, a short polite follow-up or a phone call is more reliable than waiting for a signal. The Free plan has no reminders or reply detection.