Check daily email quota
Did you know that Google limits the number of emails per-day that you can send from Add-Ons? For consumer accounts, the limit is 100. For EDU and Business accounts, it's 1500. Note that this quota affects your entire account, and isn't specific to each spreadsheet where you use Flubaroo.
Flubaroo will notify you if you've exceeded this limit when trying to send emails. But sometimes it's helpful to check before you send out a large number of grades. To check, select from the menu Flubaroo > Advanced > Check Email Quota. You'll see a window pop-up that looks like this:
If you've exceeded your quota, you'll need to wait up to 24 hours for it to be reset.