This guide provides an overview of potential errors you may encounter when sending messages through our service. Each error is associated with a specific SMTP response, characterized by a unique Category, Error Code, and Status Code. You will also find detailed explanations of these codes, including their underlying causes and practical solutions to resolve them efficiently. Leverage this resource to quickly diagnose and address any email delivery issues you may experience.

Error Categories and Definitions #

Blocked #

Your email was blocked due to a policy or rule that prevents its delivery. This could relate to mailbox quotas, email size limitations, or specific policies set by the recipient’s organization. A common example is code 554 with enhanced code 5.2.3.

DMARC Errors #

DMARC (Domain-based Message Authentication, Reporting & Conformance) errors occur when a message fails authentication checks because the “From” header does not align with validated SPF/DKIM domains. Here are some tips to resolve DMARC errors:

• Ensure the “From” address aligns with your sending domain.
• Verify that both SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) are correctly configured for your domain.

DMARC error responses appear under codes such as 509 (5.7.509), 550 (with enhanced codes including 5.0.0, 5.7.20, 5.7.26, 5.7.5, 5.7.509, 5.7.6, 5.7.8, 5.7.9, and missing), 553, 554 (5.0.0 and 5.7.5), and 555 (5.7.5). In each case the message was rejected for failing DMARC checks. Confirm that the “From” header domain matches your SPF/DKIM-validated domains.

Invalid Address #

This error indicates that the email was sent to a non-existent or incorrectly typed email address. The recipient may have provided an invalid address, or there could be a typo in the email. Utilize an email validation service to ensure the accuracy of email addresses before sending messages to resolve this error.

Invalid Address bounces span many codes, including 450 (4.1.1), 500, 501, 505, 508, 511, 521, 522, 525, 540, 542, 550, 551, 552, 553, and 554 with a wide range of enhanced codes. In nearly all cases the recipient’s email address is invalid or does not exist. One variant (550) indicates the address is invalid and already on the bounce list — revalidate the address and try again — while another notes the error originates from the recipient’s end due to a misconfiguration or outage on their MX server. A 550 (5.2.1) can also indicate the email account is inactive.

Mailbox Issues #

This type of bounce indicates that the recipient’s email address is invalid or their domain does not have a mailbox to receive emails. To ensure successful email delivery, it is important to:

• Validate email addresses before sending campaigns by using an email validation service.
• Double-check for any typographical errors in the recipient’s email address to avoid issues.
• Verify that the recipient’s domain is active and capable of receiving emails to prevent delivery failures.

Mailbox bounces appear under codes such as 450 (4.2.2), 452, 498, 511, 516 (5.1.6), 522 (5.2.2), 550 (with enhanced codes including 4.2.2, 5.0.350, 5.2.0, 5.2.2, 5.4.6, 5.5.0, 5.5.1), 552, 553 (5.3.1), and 554 (5.1.1). In each case the recipient’s domain has no mailbox and does not accept mail — send to a valid email address.

Mailbox Full #

A “Mailbox Full” bounce means that the recipient’s inbox has reached its storage capacity and cannot accept new messages. This issue is usually temporary and may resolve when the recipient clears space or their mailbox size is increased. To resolve issues with a full mailbox:

• Wait and retry by allowing some time before resending your email, as the recipient may clear their mailbox.
• Contact the recipient through alternative methods, such as phone or social media, to inform them of their full mailbox and encourage them to address the issue.

Mailbox Full bounces appear under codes such as 452 (4.1.1, 4.2.2), 550 (5.2.2), and 552 (5.2.2). The recipient’s mailbox is full and cannot accept additional emails.

Rate Limit Issues #

Rate limit bounces occur when you are sending too many emails to the recipient’s server too quickly. This can trigger the server’s anti-spam filters, causing your emails to be temporarily blocked. To address rate limit issues:

• Consider throttling your sending rate by slowing down the frequency of emails sent to the recipient, ensuring you do not overwhelm their server.
• Use email queues, which can help by dispatching messages in smaller batches over time, reducing the overall load.

If the issue persists, it may be beneficial to contact the recipient’s IT team to request assistance with whitelisting your sending domain, which can help prevent future rate limit rejections. Rate limit responses appear under codes such as 221, 550 (4.2.1, 5.2.1, 5.2.122), and 554 (5.2.121, 5.2.122). The recipient’s server has rejected your email due to excessive volume — contact the recipient through an alternative method.

Recipient Error #

These errors suggest that the recipient’s server is blocking incoming emails from external sources, preventing successful delivery. In cases where the email rejection continues, it is recommended to reach out to the recipient via an alternative communication method.

Recipient Error responses appear under codes such as 450, 502, 503, 510, 521 (5.3.0, related to Cloudflare’s email routing service), 530, 535, 541 (5.4.1), and a large range of 550 enhanced codes. Most indicate the recipient’s server denied your email and communication outside their domain has been restricted. Certain 550 codes (5.4.316, 5.4.317) indicate the message expired after exceeding the retry time limit, and 550 (5.7.705) indicates the recipient’s Microsoft 365 tenant exceeded a limit or threshold.

Rejection Error Codes #

Rejection Error Codes provide crucial insights into why emails may not be successfully delivered. Understanding these error codes can help diagnose issues with DNS records, server configurations, and email reputation.

Rejection responses appear under codes such as 450 (4.1.8), 501, 533 (5.7.1), 550, 553 (5.1.8), and 554. Most indicate the sending domain lacks valid DNS records — verify the domain and ensure all necessary DNS records are configured with your DNS provider. Some 550 variants indicate the message exceeds the recipient’s size limit (5.2.3), exceeded the maximum number of server hops due to mail loops (5.4.0, 5.4.12, 5.4.14), or a detected mail loop (5.4.6). A 554 (NA) indicates the domain or email has a poor reputation, or the MAIL FROM domain appears invalid.

Sender Reputation #

If your emails are being blocked due to potential spam or malicious content, follow these steps to improve your sender reputation and ensure successful delivery:

• Check IP and Domain Blacklists: Verify if your IP or domain is listed on any blacklists using Blacklist Check Tools. If found, follow the specific delisting procedures provided by the blacklist service.
• Ensure Proper DNS Configuration: Configure your SPF records to authorize your email servers, implement DKIM signatures to authenticate your emails, and set up DMARC policies to protect against email spoofing. Use tools like MXToolbox to validate your SPF, DKIM, and DMARC records.
• Monitor and Improve Email Reputation: Use Microsoft SNDS (Smart Network Data Services) to check the status of your IP addresses with services like Outlook and Hotmail, and access Google Postmaster Tools to track your domain’s performance and reputation with Gmail.
• Send Smaller Email Batches to Build Reputation: Avoid sending large volumes of emails from a new or inactive IP/domain. Begin with smaller batches and gradually increase the volume over time.
• Clean and Segment Email Lists: Remove inactive or invalid email addresses to reduce bounce rates. Use email validation tools to verify addresses before launching campaigns. Monitor feedback by tracking spam complaints and promptly removing recipients who report your emails as spam.
• Check Email Content and Follow Best Practices: Avoid spammy phrases such as “Congratulations!!!” or “Act Now!” in subject lines. Maintain a balanced image-to-text ratio. Ensure all links are safe and not blacklisted. Comply with anti-spam regulations by including a clear unsubscribe link in all marketing emails, adhering to laws like CAN-SPAM and GDPR.

By following these guidelines, you can enhance your sender reputation and improve your email deliverability rates.

Frequently Asked Questions #

What information does each bounce error include? #

Each error is associated with a specific SMTP response characterized by a unique Category, Error Code, and Status (enhanced) Code, along with a definition and recommended solution.

How do I fix a DMARC error? #

Ensure the “From” address aligns with your sending domain and verify that both SPF and DKIM are correctly configured so the message passes DMARC validation.

What should I do about Invalid Address bounces? #

Use an email validation service to confirm address accuracy before sending, check for typos, and remove addresses already on the bounce list.

How do I handle a “Mailbox Full” bounce? #

Wait and retry later, as the issue is usually temporary, or contact the recipient through an alternative method such as phone or social media.

What causes Rate Limit bounces and how do I prevent them? #

They occur when you send too many emails too quickly. Throttle your sending rate, use email queues to send smaller batches, and ask the recipient’s IT team to whitelist your domain if needed.

How can I improve my sender reputation? #

Check blacklists, configure SPF/DKIM/DMARC correctly, monitor reputation with tools like Microsoft SNDS and Google Postmaster Tools, warm up new IPs/domains with smaller batches, clean your lists, and follow content best practices.