Hi @abcrandy
Thank you for posting.
According to your description, you are encountering volume quota issue. Even you change flow back to original settings, it still fails. If any misunderstanding, please kindly let me know.
First of all, please help to check whether you are assigned trial plan. If you're not on a free plan or a trial plan, runs are pooled across all users in your organization.
Meanwhile, there is throttling limits for ‘Mail’ connector which only allows us sending 5 emails per 5 mins. Not sure if that’s the root cause for your scenario. But if you want send more emails within certain time period, we recommend you can try ‘send an email(V2)’ using ‘outlook Office 365’ connector if possible.

https://docs.microsoft.com/en-us/connectors/sendmail/
You also mentioned the flow fails even though you delete that additional action. Please help to delete the ‘send email notification’ action and save this flow as a copy. And run that copied one for test purpose. I’m considering maybe some corrupts happening on this flow.

Hope the content above may help you.
Best regards,
Anna