550 5.1.1 User Unknown / Address Does Not Exist: Causes and Fixes

A 550 5.1.1 bounce means the receiving server accepted the connection, looked up the address, and found no mailbox for it. The server that answered is the right one for the domain; it simply does not know that user. That narrows the causes to a short list: the address is mistyped, the mailbox was deleted, or the domain recently moved providers and the mailbox was never created on the new one. The last cause is the one that confuses people, because the address worked last month.

Quick answer

If you sent the message, check the address character by character and ask the person for it again by another channel; then remove it from any list it is on. If people are getting this error when writing to you, log in to your mail provider and confirm the mailbox exists with that exact spelling. After a provider switch, every address has to be created again on the new provider; nothing carries over through DNS.

The common wordings

• Gmail: "550-5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces."
• Postfix (most hosted providers): "550 5.1.1 <name@domain>: Recipient address rejected: User unknown in virtual mailbox table" or "... in local recipient table".
• Microsoft 365: "550 5.1.1 RESOLVER.ADR.RecipNotFound; not found" or the newer "550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup".
• Exim (cPanel hosts): "550 No Such User Here" or "550-Verification failed for <name@domain> ... No Such User Here".

If you sent the message

1. Compare the address with the source. Common slips: a dot or hyphen missing, a swapped letter in the domain, .co instead of .com, a trailing space from copy and paste.
2. Check whether the person still works there. Mailboxes are removed when people leave, and the company may not have set up a forward.
3. Try the role address instead (info@ or support@) and ask for the right contact.
4. Remove the address from newsletters and sequences. A 5.1.1 is a hard bounce; sending to it again hurts your reputation and never succeeds. The bounce handling guide covers the routine.

If people cannot reach you

1. Log in to your mail provider and check the mailbox list. The address must exist with that exact spelling; sales@ and sale@ are different mailboxes.
2. If you recently switched providers, run the domain through the free MX lookup. If the MX still points at the old provider, bounces come from a server that no longer has your mailboxes. Update the MX to the new provider.
3. If the MX is right but the bounce says "User unknown in virtual mailbox table", the new provider does not have that mailbox yet. Create it.
4. If the bounce came from your old host even though MX points elsewhere, the sender is on that same host and mail never left it. That is a routing setting on the old host; see mail looping back to the old host.
5. Send yourself a test from an outside address (a personal Gmail works) after each change.

On OquMail

OquMail delivers only to mailboxes and group addresses that exist in your dashboard. Anything else is refused at delivery time with a 550 so that senders learn quickly instead of mail vanishing. If customers guess addresses like contact@ or hello@, create those as a group that delivers to the people who should see them; a group costs nothing and does not use a mailbox slot.

Common questions

Is 5.1.1 ever temporary?

Rarely. It is a hard bounce. The one exception is the hour or two after a mailbox is created or a domain is moved, when a resolver still has the old answer cached. If the address definitely exists, wait for the TTL and try once more.

Why did a message to a valid address bounce from Microsoft 365?

Microsoft sometimes returns 5.1.10 when directory-based edge blocking is on and the address has not been synced to the cloud directory. The recipient's admin has to add the address; nothing on your side fixes it.

How do I check an address without emailing it?

You cannot be certain without sending, but a lookup of the domain's MX records tells you whether it receives mail at all. Our guide on checking an address before you send covers what can and cannot be verified.

Want to see how your own domain is set up? Use the free MX lookup: it runs in your browser and stores nothing.