Understanding and Resolving Gmail and Yahoo Email Error Codes

With Google and Yahoo transitioning several long-standing best practices to enforced sender requirements, we created the following guide to ensure you understand where you can find evidence of delivery issues and begin to understand what additional steps you need to take in order to ensure you are sending according to their guidelines.

What are error codes?

Email error codes and bounce strings are generated when one email server attempts delivery to another email server that results in a failure. Error codes are also commonly referred to as bounce codes, SMTP errors, or Delivery Status Notifications (DSN).

You can use the messages and codes to help understand the underlying reason and attempt to troubleshoot them. Most often, the source sending emails on your behalf will have developed software to handle errors in an automated fashion for you. Where some email sources may expose these errors to you through their interface, the majority don’t, or they aggregate them to more generic bounce categories.

The numerical portion of the error allows for automated rules to be reliably created and actioned against by software. The message portion is intended to give a human-readable explanation.

Why are error codes important?

Understanding error codes, their meanings and what actions to take are important for anyone sending email at scale. Though there is some consistency among several large mailbox providers, there remains wide variation of the error codes when considering the much broader email receiving ecosystem. It’s believed there are thousands of unique error codes and messages that could potentially be returned.

Depending on a number of factors, an error can be either classified as either “temporary” or “permanent.”

• Temporary Failure: When there is a failed delivery attempt, but retrying is acceptable. Example: Mailbox is full or resources are unavailable.

• Permanent Failure: When there is a failed delivery attempt and retrying will not result in success. Example: Email address does not exist or message is too large.

The context of email delivery errors and DMARC is that they offer real evidence, either to validate what can be observed through the dmarcian platform or as an alternate signal altogether. Though complete error codes and strings are not discoverable in the dmarcian console, the application provides a much easier means of identifying gaps in email authentication (DMARC/SPF/DKIM) and view across all of your email sending sources.

We’ve hand-selected the most relevant error codes that may arise from Google and Yahoo’s sender requirements or other email authentication failures (SPF/DKIM/DMARC).

Gmail SMTP Error Codes

Google has always offered useful bounce strings, written in plain English, and are one of the very few who provide links to additional, relevant information on the cause and solution.

The following are the Gmail error codes and messages resulting from issues related to DMARC, SPF, DKIM and their revised sender guidelines:

550, “5.7.26” – ” Unauthenticated email from domain-name is not accepted due to domain’s DMARC policy. Please contact the administrator of the domain. If this was legitimate mail, visit Control unauthenticated mail from your domain to learn about the DMARC initiative. If the messages are valid and aren’t spam, contact the administrator of the receiving mail server to determine why your outgoing messages don’t pass authentication checks.

The following are a few things to consider upon receiving error messages of this type:

• You will want to determine which email source sent the message and either complete the necessary configuration or troubleshoot these related settings.
• When sending at higher volumes, you may receive an occasional error, even if your SPF, DKIM and DMARC settings are accurate. If you send one million messages and received three errors of this type, the issue is likely at the receiver side and is not a reason for concern.

550, “5.7.26” – “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. Visit Prevent mail to Gmail users from being blocked or sent to spam for more information.”

550, “5.7.26” – “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. Visit Prevent mail to Gmail users from being blocked or sent to spam for more information.

Gmail’s full list of errors and codes

Yahoo SMTP Error Codes

Yahoo is another mailbox provider that has put forth considerable effort to return useful information in their response codes and strings.

The following are the Yahoo errors codes resulting from faulty DMARC, SPF, and DKIM records or other stated sender guidelines:

5XX (553 and 554) permanent errors – A 553 or 554 SMTP error indicates an email could not be delivered due to a permanent problem. Message delivery can be permanently deferred because of the following:

• You’re trying to send a message to an invalid email address.
• Your message failed authentication checks against your sending domain’s DMARC or DKIM policy.
• The message contains characteristics that Yahoo won’t accept for policy reasons.
• Other suspicious behavior which leads Yahoo to issue a permanent rejection for your SMTP connection.
• Your IP is listed by Spamhaus. Check with https://www.spamhaus.org.

Yahoo recommends to not resend an email that comes back with a 5xx error and that list managers should have a policy for removing email addresses that generate these errors.

• Authentication failures
• Your email failed one or more authentication checks that Yahoo uses to verify emails are truly sent from the domains they claim to originate from.
• Yahoo rejects emails for failing DKIM authentication when all of following conditions apply:
  • The signing domain publishes a policy which states that all emails from the domain must be signed and authenticated with DKIM to prevent forgery.
  • The signing domain is identified in the “d=” tag of the DKIM signature.
  • The rejected email couldn’t be authenticated against the sending domain’s policy, for example, due to a missing or bad signature.
• If you’re not the system administrator for the mail servers affected, contact the administrator, so they can look into the situation further.
• For mailing lists, also known as “listservs,” you should change your sending behavior by adding the mailing lists’ address to the “From:” line, rather than the sender’s address. Also, enter the actual user/sender address into the “Reply-To:” line.

Here’s Yahoo’s full list of error codes.

If you need help figuring out email error codes, let us know. You can also register for a free trial to get visibility and insight into your email domains.

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