@WebEndev,
How may subscribers do you have? I suspect you are running into the HostGator limitations as listed here.
It looks like you need to apply throttling no matter how many subscribers you have although I suspect the more subscribers you have the more likely you are to see a problem.
They recommend sending one email every 8 seconds. You may need to explore throttling (maybe WPMQ) or diverting emails through a service like Amazon SES.