Is email limit per hour each clock hour or a sliding window?

How does the email limit of 300 per hour work? Is it

1. Per each clock hour (e.g., 3:00 to 4:00, 4:00 to 5:00),

2. The number of emails in a sliding window of the previous hour from
now (e.g., if it is now 3:25, then 2:25 to 3:25),

3. When you hit the limit, you can't send for the next 60 minutes?

I reached the limit and Plesk displayed

Attempts to send mail after exceeding the limits during the reporting period:
Started At Back To Normal At Rejected Messages...

Is email limit per hour each clock hour or a sliding window?