The client and the server converse back and forth during an SMTP connection. The server responds to text commands from the client with number codes appended to messages. In our blog post on SMTP Commands and Response Codes, we covered this communication. Since error codes might imply different things on various systems, there are a number of potential solutions available to you. One of these, the 550 5.1.1 or 550 not local host error, will be discussed and fixed today.

Error - 550 not local host

We think that you have not performed authentication setup in your mail client if you see the error message "The message could not be sent...550 not local host, not a gateway."

In Microsoft Outlook and Outlook Express, you can verify the account configuration once again. In the Outgoing Mail Server configuration, you must choose the "My outgoing server requires authentication" checkbox.

What does error 550 5.1.1 mean?

A 550 5.1.1 code is often returned to the client in response to an RCPT TO command. It is a type of error code that typically denotes a problem with the recipient server. "User unknown" appears as an error message the most often. The recipient address is typically the cause of this since it is invalid. However, there are many potential causes and error signals.

You may see 550 5.5.1 error messages.

550 5.1.1: User unknown

550 5.1.1: Recipient rejected

550 5.1.1: Recipient address rejected

550 5.1.1: Recipient address rejected: User unknown in the virtual alias table

550 5.1.1 : Recipient address rejected: User unknown in relay recipient table

550 5.1.1: The email account that you tried to reach does not exist

550 5.1.1: Mailbox not found

550 5.1.1: Invalid mailbox

550 5.1.1: Mailbox unavailable

550 5.1.1: Unrouteable address

550 5.1.1: Mailbox temporarily disabled

550 5.1.1: No such user here

Bonus errors:

550 5.1.1: Protocol error

This error message could appear if the sent email was denied. In this instance, it is locating a problem with a transmitting server that is using the SMTP protocol rather than the ESMTP protocol. Does this imply that problems with the recipient and sending servers might both result in the 550 5.1.1 error code?

Causes of 550 5.1.1 and remedies

A typo in the recipient's email address, such as a skipped or repeated character, results in an incorrect address and is the primary cause of SMTP error RCPT TO 550 5.1.1. So the first item that needs to be verified should be the email. If the email address is legitimate, more investigation is required.

Problems with the receiving server

+---------------------------+------------------------------------------+ | Reason | How to solve | +===========================+==========================================+ | The email address does | - Check the email address for spelling | | not exist | > errors. | +---------------------------+------------------------------------------+ | MX record resolving to | - Check the routing and DNS | | the wrong server | > resolution. | | | | | | - Contact the recipient server | | | > administrator to edit DNS records | | | > and set up correct MX records. | +---------------------------+------------------------------------------+ | Custom security settings | - Use Telnet to connect to the | | | > receiving server. | | | | | | - If the connection fails, the email | | | > may be blocked by the recipient | | | > server. | | | | | | - Explore possible issues on the | | | > sending server. | +---------------------------+------------------------------------------+

Problems with the sending server

+------------------------+---------------------------------------------+ | Reason | How to solve | +========================+=============================================+ | Blacklisted IP | - Using a dedicated tool, check to see if | | | > the IP address is blacklisted. | | | | | | - Examine email logs for spamming | | | > activity. | | | | | | - Check the server for malicious scripts. | | | | | | - Block any suspicious email account and | | | > strengthen password security. | | | | | | - Follow the blacklist-removal process. | +------------------------+---------------------------------------------+ | Configuration issues | - Check DNS records including rDNS, SPF, | | | > DKIM, and DMARC. | | | | | | - Set up records that will ensure that IP | | | > addresses really belong to the | | | > domains they claim. | +------------------------+---------------------------------------------+

What might be in error on the other end

The email address is invalid

The email account to which you sent the message cannot be located on the recipient server. As a result, the email cannot be delivered and will bounce back to the sender.

How can this be fixed?

The only thing you can do is double-check the email address for typos.

Incorrect server resolution for an MX record

A DNS record called a Mail Exchange (MX) record is used by mail servers to decide where to send emails. To contact the recipient mail server, the sender mail server searches for the MX record. The MX record links to the A record, which links to the IP address of the mail server. For instance, the A record of, which points to, is referenced by the MX record of the domain You may experience the 550 5.1.1 error if the MX record for the recipient domain resolves to another server. Due to the recipient mail server's inability to recognize the email account in it, the email cannot be delivered.

How can we fix this?

Using the dig tool and the +trace argument, you must first check the DNS resolution and routing. As an illustration:

dig +trace

Your end should resolve the domain to the proper server. Following that, verify the domain's MX records using:

dig MX

Contact the receiving server administrator if you discover any problems so they may change the DNS records and set up the proper MX records. Incorrect DNS settings for the recipient's domain will prevent the MX from resolving properly.

Individual security settings

It's improbable that a receiver server has no anti-spam security measures in place. Your email may have been rejected by spam filters if you receive a 550 address rejected error message. It doesn't necessarily imply that you're a spammer or that your email is unsavory. Strict security regulations frequently stand in the way.

How can this be fixed?

To see if the firewall is preventing access to the sending server, telnet the receiving SMTP server (or its IP address). This was covered in our blog post How to Test SMTP Server. Run these commands:

telnet 25


telnet 25

A receiving server has blocked your emails if telnet reports that you cannot connect. Therefore, the transmitting server is to blame for the 550 5.1.1 error, which is significantly closer to home.

What could go wrong when sending the message?

As was said above, the email that was sent from your server was intercepted by spam filters. There are several causes for this.

Blocked IP

This is how it seems. Some organizations keep an eye on servers and put them on a blacklist if they are found to be spam senders. Most mail servers consult these blacklists and ignore emails arriving from certain senders. Your server is probably included on one of these blacklists if you receive the error code 550 5.1.1. Remember that using a shared IP address makes it more likely that you will end up on a blacklist. If you have a dedicated IP address and are still blacklisted, your server was probably compromised.

How can this be fixed?

Checking to see if your IP is blacklisted should be done first. Use whichever tool you like, including IP Reputation Monitor by GlockApps and Blacklist Check by MXToolbox. Following that, you must adhere to the blacklist-removal procedure. Typically, it is determined by the specifications of a certain blacklisting database.

Dedicated IP owners should check their email logs for spam activity. Additionally, you must scan the server for dangerous programs. Block the suspicious email account after that, and then tighten your password policy. Of course, remember to update the password as well.

Configuration difficulties

Configuration issues

Inadequate transmitting server verification is a different spam trigger. This relates to the reverse DNS record and the TXT DNS records (SPF, DKIM, and DMARC) verification (rDNS). If you encounter a 550 5.1.1 refusal, the DNS configuration may be incorrect.

How can this be fixed?

Using the -x flag added to the command-line tool dig (in OSX and Linux), manually perform an rDNS lookup as follows:

dig -x

The "ANSWER SECTION" of the result will display the entire rDNS record. The record needs to be set up and legitimate. Online tools are another option for rDNS verification.

  • Check the domain's TXT records. The following steps can be taken manually using dig:

dig NS

And then

dig TXT

Use nslookup as well as follows:

nslookup -type=txt

The SPF record that begins with v=spf1 must then be located. For instance:

v=spf1 ~all

  • According to the needs, update it. Make careful to generate an SPF record if there isn't already one.

  • Check your emails for the DKIM record that begins with v=DKIM1. Using an online tool like DKIM Record Check, you may verify the DKIM signature. Read DKIM Explained for more information.

  • Additionally, you must search for the DMARC record with the prefix v=DMARC1. Always, the receiving server searches for it. Its absence or any alterations will cause the entire verification to fail. Read DMARC Explained for additional details on this.

Fix for Exchange, Exim, Postfix, and Qmail servers error 550 5.1.1 user unknown

Error '550 5.1.1 User unknown' in Exchange Servers 2010 and later is primarily caused by security restrictions for Microsoft Exchange Servers. To stop spam, they authenticate the sender and by default limit email sending. All emails sent to email addresses outside of your local area are returned with the 550 5.1.1 error. The limitation is now lifted as follows:

  • Go to Recipients > Mailboxes in the Exchange admin center.

  • To set up mail forwarding, click the mailbox you want.

  • On the mailbox properties page, click Mailbox Features.

  • To modify the delivery constraints, choose View details under

    Message Delivery Restrictions.

  • Uncheck the box. Make it necessary for all senders to be verified.

    After selecting OK, click Save.

Exim servers

550 5.1.1 User unknown for Exim mail servers User unknown is not frequently a problem. But if it does, the path probably has to be determined. Check that the mail server settings are accurate first. Then, check the server's newly added custom rules and filters. You may use this to find the 550 address rejected that was denied and fix it.

For the servers, Postfix and Qmail

Verify the recipient domain resolution first, then. It needs to connect to the right server. Turn off the domain's email service if it is hosted somewhere else. Check the server's new custom rules and filters. The delivery of the letter must not be hampered.

Can I send bulk emails without encountering 550 5.1.1?

For any marketer who deals with bulk emails, 550 5.1.1 is a nightmare. Why? Just picture hundreds of people receiving your email campaign. It's very possible that many of the email addresses are invalid if you didn't validate them all. Multiple bounces with 550 5.1.1 User unknown are required in this. Will this have any impact on email hosting companies? How do you feel? Of course, it will, and your client will almost certainly be exposed as an email spammer that sends messages to shady addresses.

Maintaining a clean mailing list is the only surefire strategy to prevent this failure, as you would have anticipated. To do this, you must guarantee that validating email addresses is a high-priority task. Using the market's accessible bulk email verifiers, you may quickly automate the procedure. We've already covered some of them in the Email Testing Checklist.

To conclude

You can see that the 550 5.1.1 Unknown user is not an intractable problem. Pay attention to your server settings and make sure to validate email addresses. You won't endanger your email campaign by taking these actions. Additionally, you should check your emails for spam, which Mailtrap may do for you. In addition to having capabilities for email content verification and debugging, Mailtrap provides a service for delivering test emails through SMTP. There is such a thing as having too much good. Read the Mailtrap Getting Started Guide to learn more about what else this service provides.