Simon applies suppression at two layers. Both need to be configured for an unsubscribed contact to be reliably kept out of a send.

The two layers of suppression

Layer	What it does	Where you configure it
Segment-level exclusion	Removes unsubscribed contacts from the audience when the segment refreshes	In the segment builder, by excluding an unsubscribed segment
Send-level suppression	Applies the live suppression list at the moment of send, as a final safety check	In the channel action for each send

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You need both layers
Excluding unsubscribed contacts from your segment alone isn't enough. Segments are evaluated on a refresh schedule, so a very recent unsubscribe may not propagate in time for the next send. Send-level suppression reads the live list and catches anything the segment refresh missed.

Where Simon learns about unsubscribes

Simon records unsubscribes from:

Unsubscribe link clicks in Simon-sent emails
Suppression datasets you maintain in Simon
Sync-back from ESPs that support bidirectional suppression
Privacy request submissions

If a customer unsubscribes through a channel Simon doesn't monitor — for example, a destination-managed preference center that doesn't sync back — Simon won't know. For full coverage, enable bidirectional suppression sync with your ESP, or route all preference changes through a Simon-monitored channel.

Troubleshoot: an unsubscribed customer received a message

Walk through this checklist before contacting Support:

Check timing. When was the unsubscribe vs. the last segment refresh before the send? An unsubscribe that landed after the last refresh won't be reflected in that send's audience.
Confirm the unsubscribe source. Was it recorded in Simon, in your ESP, or somewhere else? If the source doesn't sync back to Simon, the signal never arrived.
Verify segment exclusion. Open the segment and confirm the unsubscribed audience is excluded. Excluding only one subscription category won't suppress contacts who opted out of a different category.
Verify send-level suppression. Confirm the channel action applies the suppression list at send time.
Check the unsubscribe type. Some ESPs distinguish hard unsubscribes, soft unsubscribes, and preference updates. Verify Simon received the specific type the customer used.

If all five checks pass and the contact still received the send, contact Simon Support with the contact ID, flow ID, and unsubscribe timestamp.