Understanding Email Delivery Failures: Causes and Solutions

Email delivery failures are a common occurrence that can disrupt effective correspondence. They can stem from various technical issues within the email infrastructure. In this article, we will explore the specific causes of email delivery failures, examine their relation to DMARC (Domain-based Message Authentication, Reporting, and Conformance), and find possible solutions for each cause.

Common Causes of Delivery Failures

Understanding the common causes of email delivery failures is essential for ensuring effective communication and troubleshooting issues promptly. The following are some of the causes you might encounter.

Invalid Email Address

When an email is addressed to an invalid or nonexistent recipient, the mail server returns a delivery failure response indicating that the recipient’s email address cannot be located. This could be due to typographical errors, outdated contact information, or changes in the recipient’s email system.

Gmail/Google Workspace

550-5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. For more information, go to https://support.google.com/mail/?p=NoSuchUser c12-20020a2e9d8c000000b002d0e89e4d04si186788ljj.36 - gsmtp

Microsoft (Outlook, Hotmail, Live, etc.)

550 5.5.0 Requested action not taken: mailbox unavailable (S2017062302)

Yahoo

554 delivery error: Not a valid recipient

Recipient Blocking

Recipient blocking occurs when an email server or filtering system intentionally prevents the delivery of an email to a specific recipient or group of recipients. This blocking can be implemented for various reasons.

Microsoft (Outlook, Hotmail, Live, etc.)

550 <[email protected]> spam complaint: marked your email as spam and is currently blocked.

Yahoo

Rejected: 550 <[email protected]> spam complaint via A:Yahoo!-Mail-Feedback/2.0 : marked your email as spam and is currently blocked.

Mailbox Full

When an email server detects that a recipient’s mailbox has reached its storage capacity limit, it returns a delivery failure response indicating that the recipient’s mailbox is full. This situation typically arises when the recipient’s mailbox has accumulated a large volume of emails and has reached its storage limit, preventing new emails from being delivered.

Gmail/Google Workspace

552 5.2.2 The email account you tried to reach is over quota

Microsoft (Outlook, Hotmail, Live, etc.)

554 5.2.2 mailbox full;

Reputation-related Failures

Poor IP Reputation

The reputation of the sending IP address plays a significant role in email deliverability. If an IP address has been associated with spamming or other malicious activities, mail servers may refuse to accept messages from that address, resulting in a delivery failure.

Gmail/Google Workspace

421 4.7.28 [x.xx.xx.xx] 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 rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. - gsmtp

Yahoo

421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to unexpected volume or user complaints - see https://postmaster.yahooinc.com/error-codes

Microsoft (Outlook, Hotmail, Live, etc.)

 smtp;550 5.7.606 Access denied, banned sending IP [x.xx.xx.xx]. To request removal from this list please visit https://sender.office.com/ and follow the directions. For more information please go to http://go.microsoft.com/fwlink/?LinkID=526655 (AS16012609)

smtp;550 5.7.1 Service unavailable, Client host [x.xx.xx.xx] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/0.0.0.0 (AS16012611)

Poor Domain Reputation

Similar to IP reputation, the reputation of the sending domain can impact email deliverability. If a domain has been flagged for spamming or other abusive behavior, mail servers may reject messages originating from that domain.

Gmail/Google Workspace

smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp

Apple

smtp;550 5.7.1 Your email was rejected due to having a domain present in the Spamhaus DBL -- see https://www.spamhaus.org/dbl/

Outlook

smtp;550 5.7.1 Service unavailable, MailFrom domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8002) [#.eop-nam02.prod.protection.outlook.com]

Authentication Failures

DMARC Policy Enforcement Failure

DMARC policies are used to authenticate email messages and prevent spoofing and phishing attacks. When an email fails to meet the requirements of the DMARC policy, it may be rejected by the recipient’s mail server, resulting in a delivery failure.

Gmail/Google Workspace

smtp;550 5.7.1 Unauthenticated email from example.com is not accepted due to domain's DMARC policy. Please contact the administrator of example.com domain if this was a legitimate mail. Please visit https://support.google.com/mail/answer/2451690 to learn about the DMARC initiative. - gsmtp

Comcast

smtp;550 5.2.0 Message rejected due to DMARC.

Apple

smtp;550 5.7.1 Your message was rejected due to example.com’s DMARC policy. See https://support.apple.com/en-us/HT204137 for info

Mimecast

smtp;550 DMARC Sender Invalid - envelope rejected

Proofpoint

smtp;550 5.7.0 Email rejected per sender's DMARC policy

You might also encounter several error codes in case of non-compliance to Google and Yahoo’s email sender requirements. We have explored these error codes in more detail in a separate article.

Resolving Email Bounce Errors

If you have encountered any of the above-mentioned error messages, here’s how you can fix them.

Invalid Email Address

Fix: Since the email address you’re trying to reach is invalid, there’s no immediate fix. It’s essential to remove the invalid email address from your database or utilize an email verification tool before sending emails to prevent such occurrences. Continuously attempting to send emails to invalid recipients can negatively impact your domain reputation.

Recipient Blocking

Fix: If you encounter recipient blocking, it suggests that your emails or domain have been intentionally blocked by the recipient’s server or filtering system, often due to perceived unsolicited email. If you believe it’s a misunderstanding, try reaching out to the recipient through alternative channels. Otherwise, reassess your email outreach practices, as recipient blocking typically indicates underlying issues with your email-sending strategy.

Mailbox Full

Fix: When emails bounce due to a full recipient mailbox, there’s little you can do directly. Consider contacting the recipient through other means to notify them of the issue. Alternatively, employ Email Verification tools to detect full mailboxes before sending emails, reducing the likelihood of encountering this problem.

Poor IP Reputation

Fix: If your emails are rejected due to a poor IP reputation, take action promptly. For third-party ESP users, reach out to their support or abuse desk, providing details of the affected IP address. If you manage your IP addresses, diligently track error codes, check for RBL listings, and initiate the delisting process to improve your IP reputation. Use EasyDMARC’s Reputation Checker tool to quickly identify issues and take action.

Poor Domain Reputation

Fix: Your domain’s reputation significantly influences email deliverability. Utilize services like Google Postmaster to monitor your domain reputation. If you notice a decline, investigate potential issues with email authentication and sender practices. Address any spam complaints promptly to restore your domain’s reputation.

DMARC Policy Enforcement Failure

Fix: In the event of a DMARC policy enforcement failure, consider adjusting your policy to a less restrictive setting, such as “p=none,” for monitoring purposes. This will allow you to monitor your email traffic and analyze reports to identify potential issues with email sources. Your next step will be configuring the sources with either SPF or DKIM.

EasyDMARC automatically parses all the complicated XML files into easy-to-read data. Additionally, it segments all the reports and classifies them based on their configuration settings. With EasyDMARC, you can easily navigate your unauthenticated sources and work on fixing them. With the Vendor Identification tool, you can simply click on the gear icon and fix everything by following the official documentation.

Once you’ve made the adjustments and ensured that all your outgoing email sources are compliant, you can switch to “p=reject”. However, it’s essential to keep an eye on the reports continuously, as your organization may start using new sources that need to be configured.