5.1.6 bounce: mailbox moved, no forwarding address
5.1.6Destination mailbox has moved, no forwarding addressPermanent: retrying will not help
550 5.1.6 <user@example.com>: Recipient has moved; no forwarding address
What it means
RFC 3463 defines X.1.6 as "Destination mailbox has moved, No forwarding address". The receiving server knows the address used to be valid and is telling you, deliberately, that the person is no longer reachable there. It is more informative than 5.1.1, which only says the address is unknown.
Few systems use it. Some hosting providers and universities return 5.1.6 for accounts of people who left, and SMTP basic code 551 ("User not local; please try <forward-path>") is the related reply when the server does know a new address.
The code is almost always in the permanent 5 class, because moving away is not a passing condition. A 4.1.6 would mean the move is underway and the sender should retry, which is unusual.
Likely causes
- The recipient left the organisation, and the admin configured the old address to answer "moved" instead of deleting it.
- A mailbox migration between platforms (for example on-premises Exchange to Microsoft 365) left the source object without a forwarding target.
- The recipient domain changed and the old domain keeps a rule that rejects mail with this code.
How to fix it
- Ask the person for their new address through another channel and update your contact list.
- Remove the old address from mailing lists and CRM records after the first 5.1.6, the same way you would for an unknown user.
- If you are the admin of the old system and you want mail to keep reaching the person, set up a forward or use a 551 reply that names the new address.
- During migrations, check that each moved mailbox has a mail user or contact object with a valid target address on the source side.
Checks that apply
- MX Lookup Shows whether the domain still points at the old system or already at a new provider, which explains a migration-related 5.1.6.
- SMTP Diagnostics Identifies the server that sent the moved notice so the right admin can be contacted.
FAQ
- What is the difference between 5.1.6 and 551?
- Both say the user is not here any more. A 551 reply can suggest a new address in its text, while 5.1.6 specifically says no forwarding address is known.
- Will the old mailbox ever accept mail again?
- Only if the admin reverses the move. Treat the address as dead unless the recipient tells you otherwise.