HTTP 422
Recipients Suppressed
One or more recipients are on the per-mailbox suppression list (hard bounce, complaint, or unsubscribe). Remove them via the suppressions API before retrying.
Example response
{
"type": "https://email.squibble.ch/docs/errors/messages/send/suppressed/",
"title": "Recipients Suppressed",
"status": 422,
"detail": "...",
"instance": "/api/v1/..."
}