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‘550-Unrouteable address’ – What causes it and how to fix it in Exim, cPanel and Outlook

‘550-Unrouteable address’ – What causes it and how to fix it in Exim, cPanel and Outlook

550 email errors happens due to a multitude of reasons, ranging from a non-existing mail account to a wrongly added mail server rule.

Today, we’ll discuss the error ‘550-Unrouteable address’ based on the various scenarios we’ve noticed them happening, during our Web Hosting Support services.

When an email is sent, it is routed from the sender mail server to the recipient mail server over internet. The recipient later downloads the mail from his mail server.

How email routes from sender to recipient

How email routes from sender to recipient

 

The error ‘550-Unrouteable address’ occurs due to the routing issues that happen in this mail delivery process, which can be due to either the recipient or sender problems.

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What causes the error ‘550-Unrouteable address’

The error message ‘550-Unrouteable address’ is usually shown to the sender in the bounce message he gets when attempting to send an email via mail clients such as Outlook or webmail.

From the complete bounce message, it is possible to know the exact reason for the error. Here, we’ll see the main reasons why this error happens and how to fix it.

1. Non-existent recipient domain

At times, the domain to which the mail was sent, may be non-existent. A typo in the email address can cause mail delivery attempt to an invalid domain.

Also, the recipient domain which was once existing, but got deleted or deactivated or expired, will no longer be routeable for the mail servers.

In such scenarios, the mails would not be delivered and would bounce back to the sender, giving the error ‘550-Unrouteable address’.

Solution:

Always validate the recipient email address you’ve given in the email for any spelling mistakes or errors.

Confirm that the domain is active and not expired, using the command:

whois domain.com 

Verify the DNS resolution results and ensure the domain is resolving fine:

dig domain.com

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2. Incorrect MX records for the recipient domain

If the MX records are not set properly for the recipient domain, the sender mail server would be unable to route correctly to the recipient mail server.

Any error in the DNS settings for the recipient domain can render it unreachable by the other external mail servers.

If DNS changes are made recently, it may take some time to resolve in the Internet and during that time period, the domain may be unrouteable.

Any temporary DNS resolution issues with the sender’s ISP can make it unable to resolve the recipient domain correctly and fail to route mails to it.

Mails sent in all these cases may bounce back to the sender, giving the error message ‘550-Unrouteable address’.

Solution:

Verify that the MX records are correctly set for the recipient domain, using the command:

dig domain.com MX

Try connecting to the recipient mail server to the port 25 and confirm that the connectivity is working fine, using:

telnet domain.com 25

If you notice DNS errors or missing MX records or connectivity issues to remote mail server, then contact the recipient server support and get it fixed.

3. Lookup failure for sender mail server

Some mail servers have strict security settings configured, to avoid spamming. Looking up the sender domain is one such security measure.

If the sender mail server is in any spam blacklist, many recipients reject the mails from it. Some even accept mails only from grey-listed senders.

RDNS is another security check where the mail server IP and its hostname should map with each other. A PTR record is set to route the IP address to the sender server name.

Many receiving servers will reject mail from IP addresses whose PTR record does not match the sending address. If a mail server rejects mail as it couldn’t verify the sender properly, it would bounce the mail.

550 error can also happen while sending mails from scripts without configuring proper sender address. In such cases, the mails may go with sender address as root@hostname or some other invalid address.

If the sender address is not routeable or if the sender hostname in the header doesn’t resolve correctly, the recipient will reject the mail.

The error message in the bounce mail in such cases would be “550-Unrouteable address 550 Sender verify failed (in reply to RCPT TO command)”.

Solution:

Confirm that the RDNS for the sender mail server IP is set fine, using the command:

host ip-address

Blacklisting or rejections from the recipient mail server can be confirmed by looking up the Spam blacklist and confirming with the remote mail server.

In case of blacklisting, scan the server, remove all spamming scripts and implement security measures to secure the server.

Also make sure that all the email scripts are configured with valid sender and recipient email addresses, to avoid mails going as nobody or with invalid addresses.

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4. Mail server configuration settings

In some mail servers such as Exim, there is a limit set for the number of emails a domain can send out every hour. If the sender exceeds this limit, the mails would bounce with the error ‘550-Unrouteable address’.

Wrong routing of the domain, where the mails are routing to remote server instead of local mail server, can also end up giving email error 550.

Anti-spam firewalls or certain security rules in the recipient mail server can lead to email bounces with 550 error.


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