SMTP error 5.1.1 Bad destination mailbox address

What SMTP error 5.1.1 means

There is no mailbox by that name on the receiving system. The part to the left of the @ is wrong, and the domain is not in question. It arrives almost always as 550 5.1.1, worded as user unknown or recipient not found. On Microsoft 365 the culprit is usually a typo, a deleted or unlicensed mailbox, an alias somebody removed, or an old address still sitting in the sender's Outlook autocomplete list.

Address status
Sent by The recipient's mail server, or the filtering gateway answering for it
Common

What SMTP error 5.1.1 Bad destination mailbox address actually means

Delivery got further than most failures do. DNS resolved, an MX host answered, the connection was accepted, and the rejection came at the RCPT TO stage when the receiving system looked the recipient up in its own directory and found nothing. Blaming DNS or your own mail flow is almost always wasted effort: the answer came from the organisation that owns the address. The rarer 451 5.1.1 is worth recognising, because there the lookup could not be completed at that moment rather than proving the recipient does not exist, so the message is queued and retried instead of bounced.

The mailbox specified in the address does not exist. For Internet mail names, this means the address portion to the left of the "@" sign is invalid. This code is only useful for permanent failures.
— RFC 3463

How 5.1.1 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 550 5.1.1 as the pairing for this status. The registry also lists 451 for this condition, but that belongs with 4.1.1 rather than with this code, because a reply code and the status beside it always agree on whether the failure is temporary or permanent.

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. The same condition reported as 4.1.1 is the other verdict on the identical problem, so a log showing that form has not produced a bounce yet.

What causes SMTP error 5.1.1 on a business phone system

A mistyped local partDoubled letters, a missing full stop between forename and surname, or firstname.lastname where the organisation uses initials. Business addresses are guessable enough that a wrong one still looks plausible, so read the exact string from the bounce.
A stale entry in Outlook's autocomplete cacheThe one nobody thinks of, and it is behind a great many 5.1.1 reports that make no sense otherwise. Outlook keeps a nickname cache of everyone you have written to, and for internal recipients it stores the legacy X500 address rather than the SMTP one. After a migration or a merger those values point at objects that no longer exist, so picking the name from the dropdown addresses a ghost while typing the address in full works perfectly.
The mailbox was deleted, or the licence was removedA leaver is processed, the account is deleted, and thirty days later the soft-deleted mailbox is purged. Pulling an Exchange Online licence does the same after the grace period, while customers carry on writing to the address.
An alias or a group that was tidied awayAliases such as accounts@ get removed in a clean-up, and distribution lists disappear when the team behind them is restructured. Nobody notices until a supplier who has used that address since 2019 gets a bounce.
Directory-based edge blocking rejecting an unknown addressWhere a tenant has edge blocking on, or a gateway such as Mimecast or Proofpoint holds a synchronised recipient list, unknown addresses are refused before Exchange Online sees them. A stale list rejects brand new mailboxes for hours.

How to fix SMTP error 5.1.1

Read the rejected address out of the bounce, character by characterThe non-delivery report names the exact address that failed. Compare it letter by letter against the correct one. A surprising share of these close in thirty seconds at this step.
Clear the sender's autocomplete entry and retype the addressIn Outlook, start typing the name, highlight the suggestion with the arrow keys and press Delete. Then type the address in full or pick it from the global address list. If that sends cleanly, the cache was the fault.
Confirm the recipient object actually existsFor an address on your own tenant, run Get-Recipient against it in Exchange Online PowerShell. It resolves every recipient type — mailbox, shared mailbox, group, contact, alias — so a blank result is conclusive rather than suggestive.
Add the old address back as an alias if it was a leaverWhere a departed member of staff still receives orders or invoices, put their address on a live mailbox as a secondary address, or on a shared mailbox the team monitors. Under fifty gigabytes that consumes no licence.
Check the message trace at the receiving endIf the failing address is yours, an Exchange Online message trace shows whether the service rejected the message or never saw it. Nothing in the trace at all points at a gateway in front of the tenant.

Fixing the underlying problem

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

Questions about SMTP error 5.1.1

Why does it fail from one person's Outlook and work for everybody else?

Because that person is selecting the recipient from their autocomplete dropdown while everybody else types it or picks it from the address book. The cached entry holds an old internal address that no longer resolves. Delete the suggestion, retype the address once, and the entry replacing it will be correct.

A member of staff has left and their email is bouncing. Can we still receive it?

Yes. Convert their mailbox to a shared mailbox before the deletion window closes, or add their address as an alias on a colleague's mailbox. Both restore delivery straight away, and a shared mailbox under fifty gigabytes needs no licence.

Does the sender always see the bounce, or can it fail silently?

The rejection happens during the SMTP conversation, so the sending server knows at once and reports it within seconds. It fails silently only where an application or device discards bounces, which is how a web form writing to a dead address goes unnoticed for months.

Could this be a spam filter refusing us rather than a bad address?

Possible but unusual, and the wording gives it away. A filter rejecting you on reputation or policy grounds normally answers with 5.7.1 or a more specific 5.7.x code. A plain 550 5.1.1 naming the recipient as unknown is a directory answer, not a policy one.

Related SMTP status codes

Source. SMTP error 5.1.1 Bad destination mailbox 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