5.1.6: The mailbox has moved and left no forwarding address
At a glance
- Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
- Basic code: varies; the enhanced code is the part that identifies the problem.
- Enhanced code 5.1.6: “Destination mailbox has moved, no forwarding address” in the IANA registry (RFC 3463), in the group addressing status (RFC 3463 §3.2).
The address used to work, but the mailbox has moved and the receiving server has no new address to give you. Mail to the old address will keep failing.
What it means
RFC 3463 §3.2 defines X.1.6 as "destination mailbox has moved, no forwarding address". The address was valid at one time, but mail is no longer accepted for it. The RFC says the code is only useful for permanent failures, so it appears as 5.1.6, with a basic code that varies by server.
The difference from 5.1.1 is small but real. A 5.1.1 says the mailbox does not exist. A 5.1.6 says it did exist, and the server knows the person is no longer there.
The related 551 reply
SMTP has a basic reply for the case where the server does know the new address: 551, "User not local; please try" followed by a forward path (RFC 5321 §4.2.3). RFC 5321 §3.4 describes the choices a server has when an address has changed:
- forward the message and say so with 251, or forward it silently with 250;
- refuse it and give the new address with 551, or refuse it with 550 and no address information.
The RFC adds that a server using 251 or 551 must not assume the client will update anything, and that servers supporting these replies should let administrators turn them off, because they disclose where a person went. RFC 5321 §7.4 warns that a client should be certain of the server's authenticity before it updates an address book from a 251 or 551 reply.
Common causes
- An employee left and the organization closed the mailbox but kept a record of it.
- A user moved to a new address at the same provider and the old one was retired.
- A department or role address was replaced by another one.
How to fix it
- Read the full bounce text. If it contains a 551 with a new address, treat it as a hint, not as permission: confirm the new address with the person before you use it.
- Look in your own records for a newer address for the contact, such as a later sign-up or the signature of a recent reply.
- If you run the receiving server and the person has a new address, consider forwarding for a transition period instead of refusing.
What to do with the address
Suppress the old address. The server has told you the mailbox no longer takes mail, and nothing on your side changes that. If the contact matters, reach them through another channel and ask them to subscribe again with their current address.
Related codes
See every bounce code in the dictionary.
Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.
How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.