HackerOne uses Intercom.io to communicate with hackers and customers. If you receive an email for which replies are set to go to for instance firstname.lastname@example.org, this means the message will be sent to our Intercom account. You can trust this domain. Always make sure that the domain is @hackerone.mail.intercom.io or @hackerone.com.
If you receive correspondence from us and the reply address uses a different domain name, it could be a phishing attack. If you are uncertain about the legitimacy of an email you received from us, don't hesitate to contact us and we will help you confirm if it is real.