block an exact sender prospectively in one Mailbox

curl -X POST "https://example.com/v1/mail/sender-blocks/block" \
  -H "Content-Type: application/json" \
  -d '{
    "sender": "user@example.com"
  }'

{
  "mailboxId": "f16f762c-c61b-441d-8b9c-9aff4650cbe1",
  "sender": "user@example.com",
  "blocked": true
}