Fixing Error SMTP Error (451): Failed to add recipient Print

  • Updated on 21-May-2024
  • error smtp, failed to add recipient, smtp error 451
  • 21

Resolving Error "SMTP Error (451): Failed to add recipient"


The "SMTP Error (451): Failed to add recipient" error occurs when attempting to send an email from webmail. This error is typically due to a temporary local problem caused by the server's inability to perform DNS lookups. DNS (Domain Name System) lookups are essential for resolving domain names into IP addresses, enabling the server to find and communicate with email recipients' servers.

Danger! This error prevents emails from being sent and needs immediate attention to avoid communication delays.


The primary cause of this error is the server's failure to perform DNS lookups, which prevents the email from being sent. This can happen for several reasons, including:

  • Incorrect DNS resolver configuration.
  • Temporary network issues.
  • DNS service outages or misconfigurations.


To resolve this issue, follow these steps:

  1. Access the Server's Command Line:

    • Log in to the server as the 'root' user. This can be done using SSH (Secure Shell) if you have remote access to the server.

    • ssh root@your-server-ip
  • Edit the /etc/resolv.conf File:

    • Open the /etc/resolv.conf file in a text editor, such as nano or vim. This file is where the DNS resolvers are configured for the server.

    • nano /etc/resolv.conf
      Info! You can use any text editor you are comfortable with. The example uses **nano** for its simplicity.
  • Add Google's Public DNS Resolvers:

    • Add the following lines to the file to use Google's public DNS resolvers. These are reliable and often used as a fallback when other DNS servers are not working.

    • nameserver
    • nameserver
      Success! Adding these lines configures the server to use Google's DNS resolvers, which can help resolve the DNS lookup issues.
  • Contact Network Provider:

    • It's recommended to contact your network provider to determine the appropriate nameservers to use. They may provide specific DNS resolvers that should be added to the /etc/resolv.conf file. This ensures that you are using the most optimal DNS settings for your network environment.

      Info! Using your network provider's DNS resolvers can improve network performance and reliability.
  • Save and Exit:

    • Save the changes to the /etc/resolv.conf file and exit the text editor. In nano, you can do this by pressing CTRL + X, then Y to confirm saving the changes, and Enter to exit.

    • CTRL + X
    • Y
    • Enter


After updating the /etc/resolv.conf file with the correct DNS resolvers, attempt to send the email again from webmail. The error should be resolved.

Success! The email should now be sent successfully without encountering the SMTP Error (451).


The "SMTP Error (451): Failed to add recipient" error in webmail is caused by the server's inability to perform DNS lookups. By updating the /etc/resolv.conf file with Google's public DNS resolvers or the appropriate nameservers provided by your network provider, you can resolve this issue and successfully send emails.

If you encounter any issues, please open a support ticket or start a LiveChat session using the icon located at the bottom right of the screen.

Was this answer helpful?

« Back