5.1.1 — User unknown

What 5.1.1 means

5.1.1 is a permanent delivery failure for the specific recipient. The receiving system is indicating that it cannot deliver to that mailbox. In many cases, it is a simple typo or an address that no longer exists.

Some providers intentionally return generic “user unknown” wording even when the mailbox exists, to reduce mailbox enumeration. The diagnostic text and consistency across repeated sends can help determine whether it is a real non-existent mailbox or a policy decision.

Common causes

Typo or outdated address

The most common cause is a typo in the local-part (before @) or an address that was changed or removed.

Alias removed or forwarding disabled

Many organizations use aliases. If an alias is removed, the mailbox may no longer be accepted and bounce as 5.1.1.

Domain receives mail, but mailbox doesn’t exist

The domain itself can be valid and have working MX records, while the recipient mailbox is missing.

What it looks like

Action: failed
Status: 5.1.1
Diagnostic-Code: smtp; 550 5.1.1 User unknown

Action: failed
Status: 5.1.1
Diagnostic-Code: smtp; 550 5.1.1 No such user

Next steps

If you are the sender

• Verify the recipient address carefully (especially spelling and punctuation).
• If you maintain a list, suppress the address after a hard bounce to avoid repeated failures.
• Ask the recipient for their correct address if you can reach them through another channel.
• If the destination is a company domain, check whether they recently changed brand/domain names.

If you administer the receiving system

• Confirm whether the recipient exists, is disabled, or is an alias that was removed.
• Check whether you intentionally hide mailbox existence (catch-all/anti-enumeration behavior).
• Ensure directory sync (if used) is up to date for newly created users and aliases.

Related codes

5.1.0 and 5.1.2 can indicate other addressing/routing issues. Policy rejections are usually in the 5.7.x family (for example 5.7.1). Browse the full list: /bouncecodes/.