SMTP error 551 5.1.6: destination mailbox has moved, no forwarding address

SMTP error 551 5.1.6 is a permanent rejection meaning the recipient's mailbox used to exist at

this address but has since moved, with no forwarding address configured to redirect mail

automatically. The enhanced code 5.1.6 is RFC 3463's exact definition: "destination mailbox has

moved, no forwarding address." This is distinct from 550 5.1.1 (the address never existed, or

existence cannot be confirmed) - 551 5.1.6 specifically asserts prior knowledge that the mailbox

was real and has relocated.

Fault: neither side, in the ordinary sense - the address was valid once; its owner has moved

on, and the receiving system knows it but cannot route to wherever they went.

Why it happens

• An employee left a company and their mailbox was deactivated without a forwarding rule

configured to their replacement or a departmental alias.

• A platform migration moved mailboxes to new addresses or a new domain without setting up

automatic forwarding from the old addresses during the transition period.

• A personal email provider account was closed or merged into another account without a

redirect being configured on the provider's side.

• A mailing-list or distribution-list address was retired in favour of a new one, and the old

address was configured to reject rather than redirect.

• A domain change following a rebrand or merger, where the old domain's mailboxes were

decommissioned in favour of addresses at a new domain, with the transition period's forwarding

eventually switched off.

In every one of these cases, someone on the receiving side made a deliberate decision to stop

accepting mail at the old address rather than silently absorbing it, which is why the reply

explicitly asserts a move rather than simply returning a generic unknown-user failure.

The transcript

$ swaks --to oldaddress@example.com --server mx.example.net
-> MAIL FROM:<sender@yourdomain.com>
<- 250 2.1.0 OK
-> RCPT TO:<oldaddress@example.com>
<- 551 5.1.6 User has moved; please try test@mailpilots.in

Some systems include the new address directly in the reply text, as shown here - always read the

full text, not just the code, since the fix may be handed to you directly.

How the big three word it

Diagnose it

There is no DNS-level or sender-side diagnostic for this condition - it reflects a deliberate

configuration choice on the receiving system. The only useful step is reading the full reply text

for a suggested replacement address.

$ swaks --to oldaddress@example.com --server mx.example.net 2>&1 | grep -A1 "^<- 551"
<- 551 5.1.6 User has moved; please try test@mailpilots.in

The fix, in order

1. Read the full reply text for a suggested new address. Many systems that return this code

specifically provide the replacement address in the same reply.

1. Update your records to the new address if one was provided, and retry there.
2. If no new address is given, contact the recipient through an alternative channel (a

colleague, a company's general contact address, a different known-good email) to find the

correct current address.

1. Remove the old address from any active sending list once confirmed - unlike a transient

condition, this will not resolve itself and will continue failing identically on every attempt.

1. If this is your own mail system and you are retiring addresses, consider configuring an

actual forwarding rule or alias for at least a transition period, rather than a flat rejection,

to avoid breaking delivery for senders who have not yet updated their records.

What not to do

• Do not retry the same address. This is a permanent 5xx; the mailbox will not reappear at

this address.

• Do not assume the recipient is unreachable entirely. The reply confirms they existed and

likely still exist - just not at this specific address any more.

• Do not guess at a replacement address pattern (first-name variations, department aliases)

without confirmation - verify through another channel instead.

• Do not treat this the same as `550 5.1.1`. The practical difference matters: 5.1.1 gives

you no information about where the person went; 5.1.6 often does.

MailPilot's bounce handling distinguishes a "mailbox moved" permanent rejection from a generic

nonexistent-address one wherever the receiving system provides that distinction, surfacing any

suggested replacement address in your delivery logs rather than only a flat failure.