551 5.1.6 Mailbox Has Moved: What the Bounce Means and What to Do

A 551 or 5.1.6 bounce means the address used to work and the receiving server no longer accepts mail for it. RFC 3463 defines 5.1.6 as a mailbox that "was at one time valid, but mail is no longer being accepted for that address", and says the code is only for permanent failures. Suppress the address now, don't retry it, and if the bounce suggests a new address, confirm it with the person before you use it.

What the standards say

Code	Standard	What it says	Read
551	RFC 5321, section 3.4 and 4.2.3	"User not local; please try <forward-path>". Servers "MAY either provide address-updating information with a 551 code" and "MUST NOT assume that the client will actually update address information"	3 Nov 2026
X.1.6	RFC 3463, section 3.2	"Destination mailbox has moved, No forwarding address": "The mailbox address provided was at one time valid, but mail is no longer being accepted for that address. This code is only useful for permanent failures."	3 Nov 2026
5.x.x class	RFC 3463	A leading 5 means a permanent failure; resending the same message won't work	3 Nov 2026

You will see the two codes together (551 5.1.6) or 5.1.6 behind a 550 reply. Read the enhanced code: it carries the meaning. Our enhanced status code entry explains how the three numbers fit together.

Why addresses "move"

• Someone left the company. Some admins set the old mailbox to reject with a "moved" message instead of deleting it.
• A domain migration. A company renames itself, moves everyone to a new domain and leaves the old one answering with 5.1.6.
• A consumer mailbox closed. The provider kept the address reserved but stopped delivery.

All three are forms of list decay. B2B lists lose addresses steadily as people change jobs; see B2B contact data decay.

What to do, in order

1. Suppress the address. Add it to your suppression list in every tool that sends, not just the one that bounced. Our suppression list entry covers why one master list beats several.
2. Don't retry. A 5.x.x code is permanent. Retrying a known-dead address only adds to your bounce rate.
3. Read the full bounce text. If it names a new address, note it, but don't import it. Reach the person on LinkedIn, by phone or through your product, and ask.
4. Look for a pattern. If many addresses at one domain bounce with 5.1.6 in the same week, the company has probably moved domains. Bounces from one domain walks through that case.
5. Re-check the rest of the list if the bounces came from an old file. Where one address has moved, others from the same export usually have too.

Hard bounce, not a sender problem

5.1.6 is about the recipient's address, not your reputation. That separates it from blocks such as Microsoft's 5.7.x codes, where re-verifying the list won't help. If you're not sure which kind you have, paste the bounce into the bounce message analyzer. For the difference between permanent and temporary failures, see hard bounce vs soft bounce, and for the closest Microsoft equivalent, 550 5.1.10 recipient not found.

Catching it before you send

Most servers that bounce a moved mailbox will also refuse it when a verifier asks, before any message is sent. That's the point of checking an old list first: the address comes back invalid and never counts against you. A few servers accept everything at the check and reject later, so no verifier will catch every case.

To pull bounced addresses out of a batch of bounce emails, use the bounced email extractor. To check a list before the next send, upload it at /verify: you get valid, risky (with the reason) and invalid files back, and the first 100 checks are free with a Google sign-in. See pricing for larger lists.