What this covers
- Duplicate sends on retry
- Opt-out checked too late
- Block sending
- Silent recovery after an interruption
Duplicate sends on retry
Any system that sends email will occasionally retry a job. Without idempotency, a retry means the same person receives the same message twice. It looks careless to the recipient and reads as spam behaviour to mailbox providers.
Send idempotency means the same step to the same person sends exactly once, regardless of how many times the underlying job runs.
Opt-out checked too late
Most tools check suppression immediately before sending. That sounds sufficient until someone unsubscribes after enrolment — they remain in the sequence, and every subsequent step depends on the check firing correctly.
Checking opt-out at enrolment rather than at send means an unsubscribed contact never enters the sequence at all. It is a smaller surface area for something to go wrong.
Block sending
Firing five hundred messages in ninety seconds does not look like a person writing emails. Mailbox providers notice, and the reputational cost lands on your domain rather than on the tool.
Staggered sending spreads volume across a window in a way that resembles human behaviour, which is what the filters are built to reward.
Silent recovery after an interruption
The worst failure mode is a run that stops halfway and then quietly restarts. The system recovers, everyone is relieved, and a portion of the list receives the message twice.
Interrupted sends should be flagged for review rather than silently resumed. Recovery that requires a human decision is slower and far less embarrassing.