Why does Sendgrid show a delivered status before a block for mailbox full errors?

Key findings

• Initial acceptance: SendGrid's "delivered" status signifies that the recipient mail server accepted the message, typically returning an 250 OK SMTP response.
• Post-acceptance processing: Many recipient mail servers perform additional checks or apply transport rules after initially accepting a message, which can lead to a subsequent soft bounce or block, such as for a full mailbox.
• Asynchronous bounces: Mailbox full errors are often transient (temporary) and may result in an asynchronous bounce notification, meaning the bounce event is reported some time after the initial acceptance.
• SendGrid's terminology: SendGrid (a Twilio company) often categorizes non-permanent bounces, like mailbox full, as "blocks" within its UI, accurately reflecting the final non-delivery for that specific message. For more information, you can read the Twilio blog post on understanding email statuses.

Key considerations

• Interpret "delivered" as "accepted": Recognize that SendGrid's "delivered" status means the email was accepted by the recipient server, not necessarily placed in the inbox.
• Distinguish bounce types: Understand the difference between immediate hard bounces (5xx errors) and delayed soft bounces or block events.
• Manage mailbox full errors: Implement strategies to handle consistent mailbox full notifications, such as suppressing these addresses from future sends to protect your sender reputation. For more insight into why these happen, read our article what causes unusually high mailbox full email bounces.
• Leverage detailed reporting: Utilize the detailed non-delivery reports (NDRs) or error codes provided by SendGrid for specific reasons behind blocks.

Key opinions

• Confusing status updates: Many marketers find it misleading to receive a "delivered" status (250 OK) for an email that is subsequently blocked or bounced due to a full mailbox.
• Expectation of immediate failure: Marketers typically expect an immediate 5xx error for any non-delivery, similar to a hard bounce, rather than a delayed block.
• Impact on reporting: The sequential "delivered" then "blocked" status can complicate the interpretation of deliverability metrics and campaign performance.
• Need for clarity: There's a strong desire for ESPs to use more intuitive terminology that aligns with the end result of email delivery.

Key considerations

• Adjust mental model: Marketers should reframe SendGrid's "delivered" as merely an "accepted" state at the receiving server's gateway. For example, emails not delivering when using SendGrid can still show "delivered" initially.
• Prioritize list hygiene: Regardless of the reporting order, consistent "mailbox full" blocks indicate an inactive or neglected email address that should be suppressed to maintain a healthy sending list. This also applies to why SendGrid shows delivered status for mailbox full errors.
• Investigate error codes: Always look for the specific non-delivery response codes, as these provide the definitive reason for the email's ultimate status. Discussions on Stack Overflow highlight this.
• Educate on email flow: A deeper understanding of how SMTP transactions and post-acceptance filtering work can help alleviate confusion.

Key opinions

• Common behavior: It is "perfectly common" for recipient mail systems to return a 5xx error at different stages of the SMTP transaction for various reasons.
• Post-acceptance rules: Many blocks, especially those due to transport rules, occur only after a message has been initially accepted by the receiving server.
• Asynchronous transient bounces: Mailbox full errors are typically transient (temporary) bounces that can manifest asynchronously, meaning the bounce notification arrives after the initial 250 OK acceptance. This is a normal, albeit sometimes confusing, part of email delivery.
• No "fix" needed: This pattern is not an indication of a problem to be resolved by Microsoft or ESPs; it's how some mail systems process messages.
• Terminology clarification: SendGrid's use of "delivered" for acceptance and "block" for a non-permanent bounce is their internal terminology, and while it causes confusion, the "block" term accurately describes the final non-delivery outcome for that message.

Key considerations

• SMTP protocol understanding: A 250 OK response signifies the successful transfer of the message from the sending server to the receiving server, not necessarily into the end-user's inbox. This distinction is critical for understanding why you see bounces followed by opens.
• Suppress problematic addresses: If an email address consistently returns "mailbox full" blocks, it should be suppressed from future mailings to prevent unnecessary sending attempts and maintain a healthy sender reputation. For more information, you can read the Twilio blog on handling blocks.
• Focus on root cause: Instead of focusing on the sequence of events, analyze the specific non-delivery response codes to understand why a message ultimately failed. This is more useful than worrying about what it means when your email is blacklisted.
• Contextualize ESP reporting: Understand that each ESP's UI might interpret and display SMTP events differently, and their definitions should be consulted for accurate data interpretation.

Key findings

• Definition of "delivered": SendGrid's documentation clarifies that the "Delivered" event is logged after the recipient server accepts the message and returns an HTTP 250 OK response. This means the email successfully left SendGrid's infrastructure and was received by the next hop.
• Mailbox full as temporary error: Mailbox full responses are typically treated as temporary errors (soft bounces) by ESPs, which may result in retries before a final status is determined.
• Post-acceptance filtering: Recipient servers often accept mail for delivery, but then sort it into other folders (e.g., spam, promotions) or drop it based on internal rules that run after the initial SMTP transaction.
• Event differentiation: Documentation distinguishes between "delivered," "bounced," "blocked," and "deferred" events, each representing a specific stage or reason for an email's journey. You can check SendGrid's support article on delivery.

Key considerations

• Understand SMTP codes: Familiarize yourself with standard SMTP response codes to accurately interpret event statuses, differentiating between initial acceptance and final inbox placement. Our simple guide to DMARC, SPF, and DKIM can help.
• Consult ESP-specific definitions: Always refer to your ESP's official documentation for their precise definitions of email event statuses, as terminology can vary slightly.
• Monitor deliverability metrics: Beyond reported delivery rates, monitor actual inbox placement rates and bounce classifications to get a true picture of your email program's performance. For further insights, read about email deliverability issues.
• Proactive list management: Regularly clean your email lists to remove inactive or problematic addresses to minimize mailbox full errors and maintain a positive sender reputation.