Bounce Error "550 5.1.1 User Unknown" or "No Such User Here" Print

What the error means

If an email you sent bounced back with a message similar to:

550 5.1.1 <name@recipientdomain.tld>: Recipient address rejected: User unknown

or a variation like "No such user here," "mailbox unavailable," or "user doesn't have an account," it means the receiving mail server accepted the connection, looked up the address you sent to, and confirmed that mailbox does not exist on their side. Your message was rejected before delivery, and it will not be retried.

This is a permanent failure (the 5xx code family means "don't try again"), which is different from a temporary deferral where the server asks the sender to retry later.

The usual causes

• A typo in the address. By far the most common cause. One wrong character in the part before the @ is enough.
• The mailbox was deleted or the person left. The domain still exists and receives mail, but that specific account was removed.
• The recipient's domain moved to a new mail provider and the mailbox was never recreated on the new system.
• You are the recipient and someone can't reach you: your domain's MX records may be pointing at a server where your mailbox doesn't exist, which often happens after a website migration or a switch to or from Google Workspace or Microsoft 365.

How to fix it

1. Check the address character by character. Compare it against a previous message from that person or their business card or website, rather than retyping it from memory.
2. Confirm with the recipient through another channel. A phone call or a message to a colleague at the same domain will quickly tell you whether the mailbox still exists.
3. If several addresses at the same domain all bounce, the problem is likely on their side: their domain's mail setup is broken or mid-migration. There is nothing to fix on your end, let them know through another channel.
4. If people tell you their mail to you is bouncing with this error, open a support ticket with us. This usually means your domain's MX records are pointing at the wrong server, and we can check and correct the routing.

Good to know

Repeatedly resending to an address that returns this error will not get the message through and can harm your domain's sending reputation. If you send newsletters or bulk mail, remove any address that returns a 550 5.1.1 from your list immediately.