Hi Peter,
It's interesting that this is how Plesk works out of the box, but we've recently replaced an email server and found that since then customers are complaining that their emails are being bounced, whereas previously their emails WERE being delivered after the hour.
This makes me wonder...
Emails used to roll over after hitting their limits on our older Plesk boxes (not sure of the version).
eg limit set at 10 emails per hour, if a user tried to send 12 emails 0900 - 1000, the 2 additional emails would be sent in the 1000 - 1100 window.
This was incredibly useful as it used to...