What does it mean when an email is “deferred”?

A deferred email is in a holding pattern, waiting for retry:

What happened:

The sending server attempted delivery. The receiving server returned a temporary failure (4xx code). The message was placed back in the sending queue for later retry.

Common deferral causes:

Rate limiting: Receiver throttling incoming messages. Server busy: Temporary capacity issues. Greylisting: Intentional delay for new senders. Temporary network issues: Connectivity problems.

What happens next:

The sending server waits according to its retry schedule (often exponential backoff). It attempts delivery again, potentially multiple times. Eventually, the message either delivers or is bounced after max retries.

Deferral is email purgatory. The message is neither delivered nor rejected, waiting for conditions to improve.