What is a transient delivery failure?

A transient delivery failure is a temporary condition that prevents email delivery at the moment but may resolve itself. Unlike permanent failures, transient failures signal "try again later."

Common causes include:

Mailbox full: Recipient's storage temporarily exceeded. Server busy: Receiving server under heavy load. Rate limiting: Sender temporarily throttled. Greylisting: Intentional delay for spam prevention. Temporary network issues: Connectivity problems between servers.

Transient failures return 4xx SMTP codes, signaling the sending server should retry. Most mail servers retry automatically over hours or days before giving up.

Transient failures are invitations to patience. The harbor is temporarily closed, but the ship should wait and try again.