Short answer: The recipient address does not exist on the receiving server. The address is misspelled, the mailbox was deleted, or the recipient domain's MX record sends mail to a server that no longer knows the address.
550 5.1.1 bounce message examples
The same code comes with different wording on different servers. In the bounce (non-delivery report) you may see one of these lines:
550 5.1.1 The email account that you tried to reach does not exist. (Gmail) 550 5.1.1 <jane@example.com>: Recipient address rejected: User unknown in virtual mailbox table (Postfix) 550 5.1.1 RESOLVER.ADR.RecipNotFound; not found (Exchange / Microsoft 365) 550 5.1.1 User unknown
Type: Permanent error. The sending server won't retry; fix the problem and send the message again.
What causes it?
| Cause | Explanation |
|---|---|
| Typo in the address | The most common cause: jon.smith instead of john.smith, a missing dot, a wrong domain ending (.co vs .com), or a look-alike character pasted from elsewhere. |
| Mailbox deleted or moved | The person left, the account was closed, or the domain moved to another provider. |
| MX points to the wrong server | The recipient changed mail providers but an old MX record still exists, so some mail goes to the old server where the mailbox is gone. |
| Alias not recreated | Role addresses such as info@ or accounts@ were not recreated on the new system. |
What to do as the sender
- Check the address character by character.
- Confirm the address with the recipient through another channel (phone, another address).
- Look up the recipient domain's MX record. If there is none, or it points to a hosting provider's default server, the problem is on their side.
- If mail to this address used to work, forward the full bounce text to the recipient's IT team.
What the receiving side's admin should check
- Make sure the user or alias exists in your mail system.
- If you switched providers, delete the old MX records and confirm every address was created on the new platform.
- If a spam filter or gateway sits in front of your server, check that recipient verification uses an up-to-date user list.
Good to know
This error is permanent: the sending server will not retry, and sending the same message again unchanged won't help. Bulk senders that hit many 550 5.1.1 bounces also damage their reputation; Gmail and Microsoft treat IPs that repeatedly mail non-existent addresses as likely spam sources. Remove bouncing addresses from your lists promptly.
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Frequently asked questions
Is 550 5.1.1 caused by a spam filter?
Usually not; it says the address does not exist. However, a gateway on the recipient side with a stale user list can return it for an address that does exist.
The message bounced but the recipient says they received it. How?
If the recipient has several MX records and one points to an old server, some messages are delivered and others bounce with 5.1.1. All MX records must be current.