The recipient is not hosted on this server and the server will not forward the message. Unlike 251 (which forwards), 551 tells the sender to retry with a different server or address, often providing the correct forwarding address.
What Does Error 551 Mean?
SMTP code 551 means the recipient exists but is not hosted on the server you connected to, and the server is unwilling to relay or forward the message. The server may include the correct address or server in its response, allowing you to redirect the message.
This code is relatively uncommon in modern email because most mail systems either accept and forward messages (251) or reject them outright (550). When it does occur, it often indicates a misconfigured MX record pointing to the wrong server, or an organizational migration where the mailbox has moved to a different server.
Common Causes
- Recipient mailbox has moved to a different server
- MX records point to a server that no longer hosts the mailbox
- Organizational mail migration in progress
- Server refuses to relay mail for the requested domain
How to Fix Error 551
- Check the bounce response for the correct forwarding address
- Verify the domain MX records are pointing to the correct server
- Update the recipient address in your records if a new address is provided
- Contact the recipient through another channel for their current email address
Frequently Asked Questions
SMTP error 551 means the recipient's email address is not hosted on the server you are trying to send to, and the server refuses to relay your message. Unlike the 251 response (which forwards the message), 551 is a permanent rejection indicating the server will not forward mail for unauthorized senders. This is commonly used as an anti-spam and anti-relay measure.
To fix SMTP 551, enable SMTP authentication in your email client's outgoing server settings by checking "My server requires authentication" and entering your credentials. Verify your SPF and DKIM records are correctly configured for your sending domain. Confirm you are using the correct SMTP server for your email account. If the error persists, check whether the recipient's server has blacklisted your IP address or domain.
SMTP 251 is a success response meaning "user not local, but I will forward your message." SMTP 551 is an error meaning "user not local, and I will not forward your message." The key difference is whether the server agrees to relay: 251 accepts and forwards, while 551 rejects the relay attempt. Error 551 is typically an anti-spam measure requiring the sender to authenticate or use the correct outbound server.