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Understanding SMTP sending errors

Sometimes when sending an email, you may receive a mail delivery failure notification or bounceback and you may wonder to yourself, “why am I seeing this?”.

This guide will cover some of the more common errors and provide an explanation of what might cause the error and any remedial steps you can take.

Let’s get started…

550 Errors

This is one of the most common error responses when a recipient mail server has rejected delivery of an email. We’ll cover some of the different causes for these errors and the error messages that can accompany the mail delivery failure notifications for 550 errors below:

  • Invalid recipient address

These errors can be returned if you’ve tried to send an email to an address that doesn’t exist, has a typo or has been deactivated:

550 No Such User Here
550-5.1.1 The email account that you tried to reach does not exist.
550 permanent failure for one or more recipients550 Requested action not taken: mailbox unavailable
550 5.7.1 <example@domain.com>... unknown user
550 5.4.1 Recipient address rejected: Access denied550 Unrouteable address.</example@domain.com>


Check for any spelling errors in the email address that you’ve sent to and check with the intended recipient to confirm if their email address hasn’t changed.

  • Relay denied/not permitted errors

These errors can indicate that recipient mail server doesn’t think that it is responsible for routing mail for the domain/address in question:

550 relay not permitted
550 5.7.1 <example@domain.com>... Relaying denied</example@domain.com>
550 relay not permitted
550 5.7.1 <example@domain.com>... Relaying denied</example@domain.com>


This could occur when the MX records for a domain are set incorrectly, when there’s a problem with the configuration of the mail server for the domain or if the email hosting for the domain has expired or been suspended.

  • Spam filtering and blacklisting issues

These errors can indicate that the mail has been blocked due to spam filter settings on the recipient server, blacklisting of the sending server IP, reputation issues or high volumes of mail originating from the sending server:

550 Sender address belongs to a non-trusted organisation 
550 #5.7.1 Your access to submit messages to this e-mail system has been rejected.
550 The sending IP (xx.xx.xx.xx) is listed on https://blacklisturl.com/.

  • SPF check failure

SPF records are used to indicate which servers are authorised to send mail for a domain. If an SPF record is misconfigured, or if an attempt to send mail from a server not authorised by the SPF record is made it can result in errors:

550 xx.xx.xx.xx is not allowed to send mail from domain.com.

  • Mailbox full or attachment limit exceeded

If the recipient mailbox doesn’t have enough available space to accept delivery of your message it can result in these errors. Most mail servers will set a maximum file size limit for attached files – if you’re trying to send an attachment larger than 10MB it can also result in this type of error.

In most cases with 550 errors it will be the responsibility of the recipient to address the problem as it will be out of your control to resolve.

If this error is being returned for a mailbox hosted with LCN, here are some things to check:

  • Check your email and hosting are active in your LCN account and that the email address has been created.
  • Check the Nameservers and MX records for your domain are correct so that mail is routed correctly.
  • Check to make sure your mailbox is not full and either delete or archive emails to free up space or alternatively you could add an email TopUp.

451 Errors

These errors are returned by the sending server rather than the recipient mail server and usually indicate that your attempt to send an outgoing mail has been blocked.

The cause will depend on the specific error that accompanies it. This could be DNS related such as no MX records. It could also mean that you have reached a sending limit for your email or domain name.

You can read more on sending limits here.

554 Errors

A 554 error is usually related to reputation leading to being blocked or blacklisted at the recipient’s side. This could be an IP, the domain name or the email address itself. To help avoid this make sure that emails you are sending are not being considered as spam to help make sure your domain and email address reputation stays in good standing.

We have a guide on how to not be considered spam here.

When this occurs, you will need to read the error to try and identify the block as it will advise who to contact to resolve the issue. For example, if it is a block implemented by Microsoft it will need to be reported to them to resolve.

551 Errors

551 is commonly an authentication error. This would usually be seen when sending emails from a mail client such as Outlook or Mac Mail or from a mobile device.

This will relate to your outgoing mail server settings likely being incorrect, this could be that you have not set up your outgoing server settings to use authentication or the details are incorrect in some way, this could be a typo in your email address or an incorrect password.

Review your outgoing server settings and make sure they are correct, you can find our settings here.

That’s it! You now have a better understanding of common SMTP errors. There are many more that are less frequently encountered – If you get an error you don’t understand please feel free to contact the support team and we will do our best to help you identify the issue.

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