Any luck, I’m in the same boat as you? From the little I’ve gained from searching online, you can check settings to see the limit of emails your newsletter plug can send in an hour. Also try setting up a external call to call your cron periodically. (not sure if that makes a lot of sense but it’s what everyone is saying).
Good luck.
P.S.: I’ll keep you posted if I find a fix for mine. Hope you’ll do the same.