EC
5.1.6
SMTPERRORNotableAddressingHIGH confidence

Mailbox has moved — no forwarding address available

Production Risk

Medium — treat as a hard bounce and suppress the old address immediately.

What this means

The recipient's mailbox has moved to a new address, but no forwarding address is available or configured on the receiving system. Delivery cannot be completed.

Why it happens
  1. 1The recipient changed their email address and the old one was decommissioned.
  2. 2An organisational domain migration occurred without setting up forwarding.
  3. 3The receiving server's forwarding rule has expired.
How to reproduce

Sending to an address that once existed but whose mailbox was migrated without a forwarding pointer.

trigger — this will error
trigger — this will error
RCPT TO:<old.user@example.com>
# Server returns:
550 5.1.6 Mailbox has moved

expected output

550 5.1.6 ...

Fix

Update contact record with new address

WHEN When bounce includes a forwarding address hint

Update contact record with new address
# Some MTAs include the new address in the DSN body.
# Parse the bounce and update your records accordingly.
UPDATE contacts SET email = 'new.address@example.com'
WHERE email = 'old.user@example.com';

Why this works

Keeping contact records up to date prevents repeated bounce events.

What not to do

✕ Continuously retry a 5.1.6 bounce

This is a permanent failure code. The old address will never accept mail.

Sources
Official documentation ↗

RFC 3463 — Enhanced Mail System Status Codes

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