Temporary failures do not block the rest of the queue

A queue worker configured with concurrency: 1 can process exactly one job at a time. When that job fails and BullMQ schedules a 32-second exponential backoff retry, the worker is blocked from pulling the next available job for 32 seconds. In a FIFO queue, every subsequent recipient in the batch waits behind the failing job. CWE-400 covers resource exhaustion via uncontrolled resource consumption — a single failing job consuming the entire worker thread is the mechanism. ISO-25010:2011 reliability.fault-tolerance requires that a fault in one component does not propagate to unrelated operations.

Set worker concurrency to at least 2 so delayed retry jobs do not block queue processing in workers/email.worker.ts:

const worker = new Worker('email', processEmailJob, {
  connection,
  concurrency: 5
})
// BullMQ moves failed-with-backoff jobs to the 'delayed' set automatically
// — they do not block other jobs while waiting

For marketing sends, add a rate limiter to prevent concurrency from overwhelming ESP rate limits:

const marketingWorker = new Worker('email:marketing', processEmailJob, {
  connection,
  concurrency: 10,
  limiter: { max: 50, duration: 1000 }
})