The recipient is not hosted on this server, but the server will forward the message to the correct destination. The email will still be delivered, but through an intermediary relay.
What Does Error 251 Mean?
SMTP code 251 indicates that the recipient address is not directly hosted on the receiving server, but the server knows where to forward the message and will do so automatically. This is common in organizations with multiple mail servers or mail forwarding rules.
While 251 is technically a success code, it adds an extra hop to the delivery path, which can sometimes introduce delays or affect authentication chains. If you see this code frequently, it may be worth verifying that the forwarding is intentional and that SPF/DKIM/ARC authentication is being preserved through the forwarding chain.
Common Causes
- Recipient mailbox is hosted on a different server within the same organization
- Mail forwarding rule is configured for the recipient address
- Legacy mail routing where a gateway server relays to internal servers
How to Fix Error 251
- No fix required - email will be delivered via forwarding
- If authentication failures occur after forwarding, ensure ARC (Authenticated Received Chain) is implemented
- Verify the forwarding chain preserves DKIM signatures
Frequently Asked Questions
SMTP 251 means the recipient's mailbox does not exist on the server that received your message, but that server will automatically forward it to the correct destination. This is a positive response indicating successful acceptance with mail forwarding. It commonly occurs when domains use cPanel mail forwarding, remote MX records, or mail gateways that relay to another server.
SMTP 251 is a success code, not an error. It confirms the server accepted your email and will relay it to the correct mail server where the recipient's mailbox resides. No action is required on the sender's part. The email will be delivered normally, though it may take slightly longer due to the additional forwarding hop.
SMTP 250 means the message was accepted for delivery at the current server, while 251 means the recipient is not local and the message will be forwarded to a different server. Both are successful responses, but 250 indicates direct delivery and 251 indicates relay or forwarding. In practice, both result in your email being delivered to the intended recipient.