How to Fix “Sender Not Authorized for Relay” (SMTP 550 5.7.367)

Getting a “sender not authorized for relay” error means your email could not be sent through the SMTP server you were using. You may see this error when sending emails from Outlook, Microsoft 365, a website, a scanner, a CRM, or another application. The error is often shown with the SMTP status code 550 5.7.367.

In simple terms, the mail server is saying, “I don’t know if you’re allowed to send this email through me.” This usually happens because of incorrect SMTP authentication, wrong server settings, missing relay permissions, connector issues, or problems with your domain’s email authentication setup. Since the server cannot confirm that the sender is allowed to use it, it blocks the message instead of relaying it.

The good thing is that this error is often caused by a configuration problem that you can find and fix. By checking your SMTP settings, sender permissions, relay configuration, and email authentication records, you can usually identify where the problem is. Let’s look at what the error means, why it happens, and how to fix it.

What Does SMTP 550 5.7.367 Mean?

The 550 SMTP status code means the mail server rejected the email. The 5.7 part usually means there is a security or permission issue. In this case, the sender may not have permission to use the server to send the email. The exact reason can be found in the full error message.

You may see messages like “Sender not authorized for relay,” “Remote server returned not permitted to relay,” or “Relay access denied.” The words may be different, but they usually mean the same thing: the mail server does not think the sender or sending system is allowed to send the email through it.

What Happens When the “Sender Not Authorized for Relay” Error Occurs

When the SMTP server receives the message, it checks whether the sender is allowed to use it for relay. If the sender does not meet the server’s requirements, the server rejects the message and returns an SMTP error such as 550 5.7.367. The message stops there and is not forwarded to the next mail server.

What Happens to the Email?

The sending application or mail client receives the rejection from the SMTP server. Depending on the system, the sender may see an error message immediately or receive a non-delivery report (NDR) shortly after. The NDR usually contains the SMTP status code and additional details that can help identify the cause of the rejection.

The email itself is not delivered to the recipient. If the rejection happens on your SMTP server, the message never leaves your mail system. If your SMTP server accepts the message but another server rejects it later, the NDR can help show which server refused it. Checking this information is important because it tells you where to focus your troubleshooting efforts.

How to Fix the “Sender Not Authorized for Relay” Error

Try these steps to fix the “sender not authorized for relay” error and get your emails sending again.

Check Your SMTP Authentication

Start by checking your SMTP authentication settings. SMTP authentication allows the mail server to confirm who is trying to send the email. If the server cannot verify the sender, it may reject the message instead of allowing it to pass through.

This is especially important when emails are sent through websites, CRMs, scanners, printers, or other applications. These systems often have their own SMTP settings, and an incorrect password or authentication method can stop them from sending emails.

To fix the problem, check the SMTP username, password, server address, authentication method, and sender address in the application. If the account password was recently changed, update the saved password as well. You should also check whether SMTP authentication is enabled for the account and whether the account still has permission to send through the server.

Check the SMTP Server and Port

Your application must connect to the correct SMTP server and port to send an email. If it connects to the wrong server or uses settings that do not match the server’s requirements, the message may be rejected.

For example, authenticated SMTP submission commonly uses port 587 with STARTTLS, but the correct port and encryption method depend on your email provider. Some services may use a different port or require a specific type of SSL/TLS connection.

Check the SMTP hostname, port, encryption, and authentication settings used by your application. Make sure they match the settings recommended by your email provider.

Also, don’t change the port at random if the current one isn’t working. The port is connected to how the SMTP server handles authentication and encryption, so using the wrong one can cause additional problems.

Make Sure the Sender Address Is Authorized

The email address in the From field must also be allowed to send through the SMTP account being used.

For example, your application may log in with: [email protected], but you use the [email protected] address in the From field:

If [email protected] does not have permission to send emails on behalf of [email protected], the SMTP server may reject the message. This restriction is an important part of email security. Without it, anyone who gets access to an authorized SMTP account could potentially use that account to send emails using other addresses in the organization.

To fix this, check whether the authenticated account is allowed to send as the address in the From field. If you’re using Microsoft 365, review the relevant mailbox permissions and mail flow settings. For third-party SMTP services, check whether the sender address or domain has been verified and whether the service has any restrictions on the addresses you can use.

Check Your SMTP Relay Configuration

If you’re using an SMTP relay, check its authorization rules. An SMTP relay may only allow certain users, IP addresses, domains, or applications to send through it.

So, check whether the IP address, sender, domain, or application sending the email is still included in the relay’s allowed settings. If your infrastructure recently changed, compare the old and new setup. A new server or network can easily change the IP address used to connect to the relay.

Note: While fixing the problem, don’t simply allow everyone to use your SMTP relay. An open relay allows unauthorized users to send email through your server and can be abused to send spam and phishing emails.

Check Your SPF Authentication

If your SMTP settings and relay permissions look correct, check your domain’s SPF record. SPF tells receiving mail servers which servers and services are allowed to send email for your domain. If a legitimate sending service is missing from your SPF record, emails sent through it may fail SPF authentication.

An SPF failure is not usually the direct cause of a 550 5.7.367 relay error. SMTP relay authorization checks whether a sender can use a particular SMTP server, while SPF is checked by the receiving server. However, both can affect email delivery, so it is worth checking SPF when troubleshooting. 

Hence, look for issues such as missing or multiple SPF records, incorrect include statements, and outdated sending services. You can use EasyDMARC’s SPF Lookup tool to check your domain’s authentication records.

End Note

The key is to treat the ‘Sender Not Authorized For Relay’ error as more than an SMTP setting problem. Your SMTP relay, DNS records, authentication methods, and mail flow rules all form part of the same email security system. When they’re correctly configured and regularly monitored, you can reduce relay errors and make it harder for unauthorized systems to send email through your infrastructure.