Welcome to distributed systems. The answer is "you can't, you can only increase the chance of the correct thing happening". It boils down to choosing between:

Welcome to distributed systems. The answer is "you can't, you can only increase the chance of the correct thing happening". It boils down to choosing between: - possibility of sending 0-1 emails - possibility of sending 1+ emails (in reality is 0+,your service could still go down permanently)