SMTP error 5.1.6 Destination mailbox has moved, No forwarding address

What SMTP error 5.1.6 means

The address used to work and deliberately no longer does. The receiving system still recognises it, knows the mailbox behind it has gone, and has no forwarding address to redirect to. That is more informative than a plain unknown user, because it confirms you had the right address once. The usual story is a leaver whose mailbox was converted or removed, or a migration that left no redirect behind.

Address status
Sent by The recipient's mail server, where it has kept a record of the old address
Rare

What SMTP error 5.1.6 Destination mailbox has moved, No forwarding address actually means

This code carries a piece of history other addressing failures do not. Rather than denying the recipient exists, the receiving system says the address was valid at some point, mail is no longer accepted for it, and there is nowhere else to send the message. Your contact record is out of date and no automatic redirect is coming to rescue you. It stays rare because most platforms answer with a straightforward unknown recipient once a mailbox is gone, and only systems configured to track retired addresses distinguish the two. RFC 3463 permits it only as a permanent failure, so the message bounces immediately.

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.
— RFC 3463

How 5.1.6 appears in a bounce

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

RFC 3463 is explicit about which verdicts this code can carry: “This code is only useful for permanent failures.”

What causes SMTP error 5.1.6 on a business phone system

A leaver processed without a forwardSomeone departs, the mailbox is closed on their last day and no redirect goes to a colleague or team address. Anyone who deals with that person occasionally — an annual supplier, an accountant — finds out much later.
A mailbox converted to shared and later removedConverting a leaver's mailbox to a shared one is good practice, but shared mailboxes get cleared out in their turn. The address goes with the mailbox, while the organisation still remembers it existed.
A migration that moved mailboxes without a redirectA move between tenants, a change of domain after a rebrand, or consolidation onto a parent company's domain. Where old addresses are not kept as aliases on the new mailboxes, everything sent to them stops at the boundary.
A forwarding rule removed once it was thought unnecessaryRedirects set up at the time of a departure are often given an end date, or quietly deleted during a tidy-up. Mail arriving perfectly well last week begins to bounce with nothing else having changed.
An address deliberately retired to stop spamLong-lived generic addresses that have been scraped for years accumulate a great deal of junk. Retiring rather than filtering is a legitimate choice, and it produces this response for anybody still writing legitimately.

How to fix SMTP error 5.1.6

Find a current address for the organisation rather than the personUse the main enquiries address from their website, or a colleague you already correspond with, and ask who covers the work now. Resending to a guessed variation of the old address rarely works.
Update your contact record and any list holding itThe same dead address sits in your CRM, in shared contacts, in a distribution group and in several autocomplete caches. Fix all of them at once or the bounce reappears next month from someone else.
If the address is one of yours, add it as an alias on a live mailboxIn Exchange Online, add the retired address as a secondary SMTP address on a shared mailbox the team monitors. Delivery resumes at once and you can see who is still writing to it, which tells you when it is safe to let go.
Build a redirect into your leaver processRedirect to a manager or team mailbox for a defined period, and review before removing rather than deleting on a schedule. Six to twelve months catches the annual correspondence a thirty-day window never will.
Keep old domains alive after a rebrand or acquisitionRetain the registration and keep the old addresses as aliases for at least a year. It costs very little against the value of the correspondence that would otherwise bounce.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.1.6

Does this mean the person has left the company?

Very often, but not always. A mailbox can be retired because of a rebrand, a domain consolidation or a decision to abandon a spam-laden address. The code tells you the address is closed, not why. If the correspondence matters, ask the organisation rather than assuming.

Can we make our own leavers' addresses do this instead of going quiet?

Most platforms will not distinguish it for you, and it is not usually what you want. A redirect to a shared mailbox beats an informative bounce, because the message reaches somebody who can act on it. Reserve outright closure for addresses you want to stop receiving anything at all.

Is there any chance the message will be delivered later?

No. The specification restricts this code to permanent failures, so the sending system returns the message rather than queueing it. You need a different address; waiting or resending to the same one will not change the outcome.

Related SMTP status codes

Source. SMTP error 5.1.6 Destination mailbox has moved, No forwarding address is defined in RFC 3463 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes