Why do some response times say 00:00:00?

If you see a response time displayed as 00:00:00, it means that both the inbound email and the reply occurred outside of your configured business hours. Because no business-hour time elapsed between the two events, the system records the response time as 00:00:00. This prevents agents from being penalised for replying outside of business hours.

These 00:00:00 response times are excluded from your overall response-time statistics.

For example, if your business hours are 9:00–17:00 and an email arrives at 08:50, then your agent replies at 09:20, the system will record a 20-minute response time, as only 20 minutes of business-hour time have passed.