550 5.1.1 Permanent Failure

User unknown / mailbox does not exist

The single most common hard-bounce code — the receiving server has confirmed no mailbox exists at this address.

Common causes

The address was mistyped.
The mailbox was deleted (employee left the company, account closed).
The address never existed in the first place.

What to do

Suppress the address immediately and permanently. This is a definitive, authoritative signal from the receiving server — there is no valid reason to retry.

FAQ

Can a 550 5.1.1 be wrong?

It's rare but not impossible for a misconfigured receiving server to return this incorrectly. In practice it's treated as close to ground truth, which is why it's the response verification tools are specifically trying to elicit.

Other permanent failure codes

550 5.7.1

Message rejected due to policy (spam/content)

551 5.1.6

Mailbox has moved, no forwarding address

552 5.2.2

Mailbox full (permanent quota failure)

Catch These Before You Send

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