Troubleshoot SMTP Error 550 "Relay Denied"

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Learn how to resolve the common email error SMTP 550 "Relay Denied." This error most often happens because of a failure to correctly authenticate with the SMTP server. Learn more about SMTP authentication methods, and how to resolve this error.

Overview of SMTP Error 550 "Relay Denied"

The SMTP error code 550 means that the recipient of the email could not be reached. This code can come up for many reasons, including an incorrect email address.

The 550 error "Relay Denied" means that the SMTP server did not recognize you as a valid sender. The most common cause of this error is a failure to authenticate correctly with the SMTP server. This can happen if:

  • The email client (e.g. Microsoft Outlook or Mozilla Thunderbird) is not set up correctly for SMTP authentication.
  • The SMTP password is not entered correctly.
  • The email client is not using the correct protocol (like SMTP-AUTH) for sending email.

The best way to troubleshoot this issue is to go through the set-up process for the email client again, being sure to check and configure the correct settings for the email server.

Check the SMTP Authentication Method

The first step to troubleshooting an SMTP error 550 "Relay Denied" is to check the authentication method for the SMTP server. For example, some servers require users to log in to SMTP with a username and password, while others require you to connect to POP3 first before sending email.

Next, go through the set-up process for the email client, being sure to set the correct SMTP authentication method. Many email clients have an "autodiscover" or "autodetect" feature for SMTP setup. This is a reliable method for setting up SMTP authentication on an email client, but if the SMTP error continues, you may need to set up the email client manually.

There are several different possible authentication methods for SMTP servers:


  • None
  • Plaintext password.
  • Encrypted password.
  • Kerberos/GSSAPI
  • NTLM

You will want to verify which method your SMTP server uses.

It may seem obvious, but if the SMTP server requires a password, be sure to enter the password correctly. For complicated passwords, you can type the password into a text editor like Notepad, then copy and paste the password into the email client.

Verify the SMTP Port

The standard SMTP port is 25, and this is the default SMTP port used by most email clients. However, SMTP services can be configured for other ports, and you will need to configure the e=mail client accordingly.

The most common SMTP ports are:

  • 25
  • 587
  • 465
  • 2525

If you are sending email from behind a firewall, you will need to verify that the corresponding port is open on the firewall as well, in order to allow the traffic to pass.

Troubleshoot the SSL Method

Most email clients have an "autodiscover" or "autodetect" feature for SMTP setup. This is usually reliable, but if the SMTP error continues, you will need to manually check the SMTP authentication method.

There are many possible combinations of SSL and authentication available for SMTP servers. The current most common variations are:


  • None

You will need to verify which SSL method the SMTP server is using, and configure the e=mail client accordingly.

IP Address Filtering

If you receive the SMTP error when you send email from one location, but not from another, and you are using the same email client, the SMTP server may be set up to only authorize users from a particular IP address or address range.

For example, if you have a laptop with Outlook configured, and are able to send email from home but not from work, IP address filtering may be to blame.

This can also be caused by a firewall issue. Many firewalls are set up to block SMTP traffic by default, to prevent spam.