As I experienced yesterday (learn the long way…), when you reach the limit of emails sent with Sendgrid, all workflows using email are simply not running and rejected ‘as an error’ by Bubble.
To prevent that, I recommend isolating each sending email action inside of a scheduled workflow. It will be faster to diagnose and will prevent sending a bug report to Bubble 
Best
From your experimentation, did SendGrid send any sort of warning email (ie. you are about to exceed your email allotment?)
As a related solution, I believe SendGrid will let you exceed the email allotment on your plan on a metered basis. (See SendGrid pricing here).
Yes, they have sent an email that you reach your limit during the day. They let you continue for couple of hours until midnight… and everything stopped 
I fixed the problem with couple of thousands more emails in my account. But it was an interesting situation that Bubble plugin was not executing any workflows containing Sendgrid. It was tough to detected.
My goodness. I’ve been ignoring Sendgrid quota warning as I am totally busy testing my app. This sounds scary though. Will keep an eye on what happens to my workflows…Indeed is a hard catch otherwise.
Thanks @JohnMark
