SMTP sending error codes and what they mean
If your email fails to deliver, you might receive an error message containing a code. This code, called an SMTP (Simple Mail Transfer Protocol) error code, provides crucial insight into the reason behind the delivery failure. The table below will help you interpret these codes and resolve the issue.
SMTP 400 – 499 error codes
A 400 – 499 error code is returned when the recipient’s email server temporarily rejects an email due to a technical problem or when an email is intentionally delayed as part of anti-spam strategies, e.g. greylisting.
A correctly configured email server should retry sending a message if a 400 – 499 error code is received. Rest assured, our email servers will automatically retry sending your message at preset intervals if it encounters a temporary error. No action is required on your end.
While the email remains in the sending queue on our email servers, you may receive automated delivery delay email notifications. These notifications inform you that the email remains in the queue, but we’ll keep trying to deliver it. The reason for the delay will also be contained within this notice. If we can’t deliver the email after six days, it will be permanently rejected and you’ll receive a delivery failure notice.
SMTP 500 – 599 error codes
A 500 – 599 error code indicates a permanent failure. These errors generally mean that the email server cannot process the request. The table below outlines these failures and provides you with recommended solutions.
| Code | Reason Given by Sending MTA | Description | Recommended solution |
| 550 | Message contained spam content | The message contains content that mostly appears in spam messages. | Change the structure of your email and try again |
| 550 | Message contained unsafe content | The message contains malware. | Send the email without the attachment or upload it to cloud storage (DropBox, Google Drive) and share it that way. Alternatively, put the attachment in a password-protected zip file. |
| 550 | A URL in this email is listed on https://spamrl.com/. Please resolve and retry | A URL in the body of the message is in the SpamRL database, which means the URL appeared in known spam messages. | Temporarily whitelist the URL by following these steps. Alternatively, remove any URLs from the body of the message and send it again.
If the URL in question belongs to you, then it’s a good idea to see if there is any suspicious content on the website, e.g. phishing page. |
| 550 | Date header far in the past/future | The date set by the email client was too far in the past or the future. | Ensure the date and time on the device sending the email is set correctly. |
| 550 | High probability of spam | The filter thinks the email is spam due to a combination of factors that pushed the email over the spam score threshold. | Change the structure of your email and try again. |
| 550 | Maximum line length exceeded (see RFC 5322 2.1.1) | A line in the headers contains more than 998 characters with no breaks. | Contact us to verify which header is too long. |
| 550 | Unroutable address | The recipient’s email address doesn’t exist. | Ensure that the address(es) you are sending to is/are correct. |
| 550 | An email address in this message is listed on https://www.surbl.org/lists | An email address that’s within the message content is listed in the Surbl blacklist. | Delist the email address or remove any email addresses from the body of the message and send it again. Note: The owner of the email address should be the one to delist the email address. |
| 550 | Relay not permitted | The email client attempting to send the email is not authenticating. | Ensure that the email program you are using to send the email has the correct settings for outgoing email. |
| 550 | This message has been reported as spam by other users | Many other SpamExperts users have reported a message that looks like this one as spam. | Change the structure of your email and try again. |
| 550 | Too many MIME parts | When sending an email, your mail application will create a string of letters and numbers within the mail header. These are known as MIME parts. Email attachments and the number of emails in the thread contribute to the number of MIME parts of an email. | Copy the content of your email response and send it as a new email, removing the existing email thread. This will generate new email headers, resulting in fewer MIME parts and improving the chances of the email being delivered. |
Please contact us if the recommended solutions above don’t solve the problem you are experiencing.
