Not sure what to tell you, I have created a newsletter 'create and send' functionality in a couple of clients' site Admins (see images 1-4).
My hosts say:
The figure is calculated every 15 minutes based on the last hour so if you sent 1000 messages 45 minutes ago then just after another 15 minutes that figure should be cleared and you can send another 1000.
We suggest customers use some kind of mailing software that has rate limiting options but failing that then split yours lists up and you should be ok.
Everything seems to work as it should. Am using DataAssist (with HTML Editor) and UE4.