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550 Relay Not Permitted – How to fix it in Exim, cPanel, Outlook, Thunderbird and Outlook Express

550 Relay Not Permitted – How to fix it in Exim, cPanel, Outlook, Thunderbird and Outlook Express

‘550 Relay Not Permitted’ is a common error message in email programs when users attempt to send mails without proper authentication. This is a security measure to protect mail server from open relay.

In our role as Dedicated Support Specialists for web hosting companies, we see this error often reported by website owners.

 

What causes the error ‘550 Relay Not Permitted’?

All legitimate mail servers require their users to authenticate with a username and password before a mail is accepted for delivery.

This prevents something called “open relay spamming” where a spammer can blast thousands of spam through a server that has broken or no authentication.

Open relays often lead to spamming and cause the mail server to be blacklisted.

550 Relay Not Permitted‘ email error usually happens due to improper user authentication. There are a few other causes too, we’ll discuss them one by one.

 

1. User not authenticated properly

To prevent spammers or unauthorized users from abusing it, every mail server has an authentication system configured in it.

Only users who authenticate in the mail server by providing their username and password, will be allowed to send mails using that mail server.

This authentication process is usually done in the email program or mail client. The option to enable authentication varies with the mail client used – Outlook, Thunderbird, Outlook Express, etc.

If users try to send mails from their mail program without authenticating their account first, the error ‘550 Relay Not Permitted’ will be shown.

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2. SMTP port blocked

By default, the SMTP port is Port 25. But some ISPs and mails servers block this port to prevent spammers from misusing or attacking that port and abusing the server.

In many cases, the ISPs or mail servers who have blocked port 25, may have configured another email port for sending mails, which is usually 587.

There are some ISPs or mail servers who implement advanced security measures such as blocking certain IP ranges and allowing users to connect only from white-listed IPs.

To know if there is a connectivity issue or port/IP block in the mail servers, use the command:

telnet domain.com 25

In email clients, there is an option to provide the SMTP port. When users try to connect to a blocked SMTP port or IP to send mails, they end up seeing the error ‘550 Relay Not Permitted’.

 

3. MX configuration issues

Incorrect MX record settings or DNS resolution issues for the recipient domain, will also cause the error ‘550 Relay Not Permitted’ when you send mail to that domain.

This usually happens when the primary MX records of that domain may be resolving to some remote mail server or when recent changes in DNS records have been done.

To verify the MX records for a domain, use the command:

dig domain.com MX

Improper settings of authentication servers such as POP, IMAP, Dovecot, etc. can also be a reason for the error ‘550 Relay Not Permitted‘.

 

4. Routing issues in mail server

Sometimes, incorrect routing configuration settings in the mail server can lead to mails being routed wrongly. This can cause remote delivery attempts for local mails and vice versa.

For instance, in cPanel servers with Exim, if a domain in listed in the ‘local domains’ list, the email delivery would be attempted locally even if its mail server is remote.

If the routing issue is in the recipient server, the error message in the bounce mail that reaches the sender would be ‘Remote server replied: 550 relay not permitted’.

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5. Apps that modify recipient addresses

Some mail programs can contain filters or forwarders which modify the email address or forward them to another address. For instance, Google Apps has a feature where it modifies the ‘To’ address of emails during email forwarding.

We’ve seen cases where wrongly set forwarders can cause mails to go into an infinite loop. To prevent email spoofing, some mail servers have security settings where such features are not supported.

During such scenarios, the email senders can end up facing the error ‘550 Relay Not Permitted’.


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