Published on 2022-06-13· Updated on 2023-09-26
We frequently encounter delivery and deliverability challenges in email campaigns such as rejections, drops, and messages landing in spam folders. Often, these issues go unnoticed until they've already caused damage to campaign outcomes or domain reputation. However, by monitoring the SMTP response codes, you can identify potential problems earlier in the process and fix them.
In this tutorial, our emphasis is on response codes associated with spam-related issues. The information below will empower you to take swift action when something is amiss with your emails or sending domain.
Let’s start with understanding the error code format.
An SMTP response code comprises of three digits:
Below, we've outlined common Gmail SMTP error codes and their corresponding solutions to address the issues:
Error code 421 4.7.0 - Not in the whitelist, Authentication issue, Spam
421-4.7.0 This message does not have authentication information or fails to pass authentication checks. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/81126#authentication for more information.
You may have issues with your SPF, DKIM, and DMARC alignment, which is a part of Gmail's email authentication. Gmail has a strict policy related to these authentications to protect users from malicious activities.
421-4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily blocked. For more information
Your domain is being blocked because it is identified as spam due to a drastic increase in email sending rates.
In case you have a new email-sending domain that is less than a year old, then it is not wise to send millions of emails on the first go. This is a sure way of getting marked as spam email. You need to increase your email volume gradually through warmup. You can take your ESP’s help in this.
IP not in whitelist for RCPT domain, closing connection.
You can stop sending emails to the user. In case you still want to send an email to them, you can get in contact with the user for the whitelisting of your email.
TLS required for RCPT domain, closing connection.
You might have seen a lock symbol at the left corner of your email. The text beside that lock should say a TLS encryption.
It means the email you are receiving is encrypted and cannot be read by anyone during transmission. This is a critical security protocol that Gmail expects the email-sending server to perform for all of the emails sent.
Error code 550 5.7.1 - Blocked, Marked as spam, Authentication
550-5.7.1 Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked.
550-5.7.1 [XXX.XXX.XXX.XXX] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been blocked.
Your emails are being considered by the Gmail anti-spam filters as spam. This might affect all or only a few of your emails.
550, "5.7.1": The IP you're using to send mail is not authorised to send email directly to our servers. Please use the SMTP relay at your service provider instead. For more information, visit 'The IP you're using to send email is not authorised...'.
This could happen if you are using an open SMTP server. There might also be an issue with IP Blacklisting in Gmail. Check the Google Postmaster tool.
Our system has detected that this message is 421-4.7.0 suspicious due to the nature of the content and/or the links within. 421-4.7.0 To best protect our users from spam, the message has been blocked.
The spam checker has identified the email content as malicious. You will need to update the email content or subject line to fix this.
You can check our Email Content Guide for a detailed tutorial on how you can fix spam identification and things to keep in mind while writing email content.
550, "5.7.1": The user or domain that you are sending to (or from) has a policy that prohibits the mail that you sent. Please contact your domain administrator for further details. For more information, visit Sorry, a policy is in place that prevents your message from being sent.
Your email is blocked due to end-user policy, and there’s no immediate fix to this.
You can either stop sending the email to the user or reach out to them for the policy modification.
The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. For more information, visit limits for sending & getting mail.
To keep Gmail accounts efficient and safe, Google limits the number of messages its users can receive.
Here is the email frequency that Gmail has set for all users (These limits are subject to change):
Per-minute: 60 Emails
Per hour: 3,600
Per day: 86,400
Size limit: A Gmail user can receive emails of up to 25 MB.
Attachment limit: A Gmail user can receive up to 500 attachments per single email.
Error code 550 5.7.26 - DMARC policy failed
Unauthenticated email from domain-name is not accepted due to domain's DMARC policy. Please contact the administrator of the domain-name domain. If this was legitimate mail please visit Control unauthenticated mail from your domain to learn about the DMARC initiative.
Your DMARC policy has failed specific checks by Gmail, and you will need to fix them. It may be an alignment or record syntax issue.
Error code 550 5.7.26 - SPF DKIM policy alignment failed.
This message does not have authentication information or fails to pass authentication checks (SPF or DKIM). To best protect our users from spam, the message has been blocked.
This message fails to pass SPF checks for an SPF record with a hard fail policy (-all). To best protect our users from spam and phishing, the message has been blocked. Please visit Prevent mail to Gmail users from being blocked or sent to spam for more information.
Your SPF is failing validation.
Hope you have some good takeaways from this step-by-step tutorial on Gmail Spam SMTP error codes and their solutions. If you have any queries, feel free to reach out to us at [email protected], and we will be glad to assist.
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