An address that hard-bounced or filed a spam complaint is added to the suppression list, and later sends to it are refused before they reach the wire. This is not us being cautious on your behalf — continuing to mail an address that has bounced is precisely what destroys a sending reputation.
Checking one address
The Suppression page has a lookup box, and GET /v1/suppression returns the list via the API. Addresses are stored hashed, so a lookup is by address and never exposes the list to browsing.
Overriding
POST /v1/suppression/allow adds an address to your workspace allowlist. It requires an attestation that you have a prior relationship with that recipient and a reason, both of which are recorded. A hard bounce means the mailbox does not exist — an allowlist entry does not conjure it back — so this is for the complaint case where the customer has genuinely asked you to resume.
Open suppressionComplaints are watched continuously across your whole workspace, and a sender crossing 0.3% is auto-suspended — well below the 0.5% that the upstream provider would act on. You would rather we paused you than that.