Why do SNDS RCPT commands not match DATA commands without evidence of bounced emails?

Key findings

• SNDS vs. local logs: SNDS reports on how Microsoft's systems perceive your sending activity, which may not always align perfectly with your internal email service provider's (ESP) delivery logs. Your ESP might report a message as delivered based on the initial acceptance by Microsoft's server, while SNDS metrics offer a deeper insight into Microsoft's internal processing and acceptance of the actual message content.
• Multiple RCPT commands: A single email session can involve multiple RCPT commands for a single DATA command if the same message is sent to multiple recipients (e.g., in a bulk send or to distribution lists). This can lead to a higher RCPT count than DATA if your SNDS view aggregates differently.
• Silent drops: Emails can be silently dropped or filtered by recipient servers without generating a bounce notification. This means your logs show a successful RCPT command, but the DATA command might not fully materialize or be counted for various reasons (e.g., content filtering, low reputation).
• Temporary errors and retries: Soft bounces or temporary rejections (like greylisting) can lead to multiple RCPT attempts for the same message, inflating the RCPT count in SNDS relative to the ultimately accepted DATA commands. This is especially relevant if an email is rejected by the mail server temporarily.

Key considerations

• Understand SNDS data: Familiarize yourself with how SNDS metrics are calculated and what each command truly represents from Microsoft's perspective. It's often a more granular view than typical ESP logs.
• Check multi-recipient sends: Review your sending patterns to identify if a significant portion of your mail involves sending a single message to multiple recipients in one SMTP session. This can naturally lead to a higher RCPT to DATA ratio.
• Monitor reputation: A discrepancy could signal underlying reputation issues. Poor sender reputation can lead to silent filtering by recipient servers, where messages are accepted but not delivered to the inbox. Consistent monitoring of your sender reputation is crucial to prevent such issues, as well as checking if SNDS data contradicts deliverability reports.
• Analyze bounce logs: Even if no explicit bounces are reported, look for soft bounces or deferred delivery notifications that might indicate temporary rejections influencing the command ratio.

Key opinions

• Data interpretation: Marketers frequently find it difficult to interpret why SNDS command counts would not match their high delivery rates in internal logs, suggesting a potential gap in understanding the nuances of how receiving servers process and report email commands.
• Impact on campaigns: Concerns arise that discrepancies might indicate silent filtering or junk folder placement, which directly impacts campaign effectiveness and ROI, even if no explicit bounces are received.
• Inconsistent reporting: Some marketers express bewilderment when seemingly normal sending behavior results in unusual SNDS reporting, highlighting a desire for clearer explanations from recipient postmaster tools.
• Focus on deliverability: The primary goal for marketers is inbox delivery, and any data indicating potential delivery issues, regardless of bounce status, triggers investigation into unseen filtering or blocklisting.

Key considerations

• Validate sending patterns: Marketers should verify if their sending practices, such as sending a single message to multiple recipients in one go, could explain the higher RCPT count in SNDS. This is a common and legitimate reason for such a disparity.
• Deep dive into logs: Beyond simple bounce rates, analyzing verbose delivery logs for temporary errors, deferrals, or specific response codes from Microsoft can provide clues to unseen delivery issues.
• Sender reputation awareness: Even without bounces, poor sender reputation can lead to filtering. Marketers should actively work on improving their domain reputation and IP reputation to avoid silent suppression of emails. This also involves understanding how SMTP errors can be interpreted.
• Engage with postmaster tools: Regularly consult SNDS and other postmaster tools for detailed insights, as they offer unique perspectives on how recipient systems are handling your mail traffic.

Key opinions

• SMTP session structure: Experts confirm that a single SMTP session can involve one DATA command with multiple RCPT commands if the email is addressed to several recipients, naturally inflating the RCPT count in some reporting systems.
• Soft bounces and deferrals: The RCPT count might include attempted deliveries that resulted in soft bounces or deferrals, where the message was not immediately accepted (and thus no DATA was fully processed or counted) but might be retried later.
• SNDS definition variability: Microsoft's own interpretations of RCPT and DATA in SNDS can vary, with some explanations suggesting RCPT indicates attempted recipients and DATA indicates actual delivered messages, or that RCPT includes soft bounces while DATA is for accepted mail.
• Reputation impacts: Lower reputation can lead to behaviors like greylisting or silent discarding, where the initial RCPT might be accepted but the subsequent DATA command or message processing is stalled or terminated without a bounce.

Key considerations

• Analyze SMTP conversation: For true insights, it is necessary to examine the full SMTP conversation logs (including response codes after RCPT and DATA commands) to understand exactly how Microsoft's servers are reacting to your mail.
• Differentiate metrics: Recognize that SNDS provides distinct metrics that complement, rather than duplicate, your internal ESP delivery reports. These discrepancies are often due to differing definitions or stages of email processing. This requires an advanced understanding of email authentication.
• Check for greylisting effects: If a significant portion of your mail is sent to new or low-volume recipients, greylisting can temporarily defer delivery, leading to multiple RCPT attempts before eventual DATA acceptance.
• IP reputation management: Consistently high RCPT-to-DATA ratios without bounces can sometimes be an early indicator of deteriorating IP reputation, where mailbox providers are more hesitant to accept the full message content.

Key findings

• SMTP protocol flow: Standard SMTP behavior dictates that the DATA command follows the RCPT TO command. For a single email, there can be multiple RCPT TO commands (for multiple recipients) followed by a single DATA command containing the message content.
• Command dependencies: The successful execution of the DATA command is contingent on the acceptance of the preceding RCPT TO command. If a RCPT TO is accepted, but the server decides to reject the message during or after the DATA command, it can result in a mismatch of reported counts without a traditional bounce.
• Microsoft's internal processing: SNDS data reflects Microsoft's internal processing and filtering, which might include pre-DATA checks or post-DATA filtering. This can lead to cases where RCPT commands are acknowledged, but the actual message data isn't fully processed or delivered to an inbox due to reputation or content filters, without a bounce message being returned to the sender.
• Error codes and sequences: SMTP error codes, such as a 503 indicating an invalid command sequence, highlight the importance of proper command order (e.g., RCPT before DATA). While these typically result in bounces, more subtle internal rejections might not manifest as standard bounces.

Key considerations

• RFC compliance: Ensure your mail server strictly adheres to RFC standards for SMTP command sequencing. Any deviation, however minor, could be interpreted differently by recipient systems like Microsoft's and lead to reporting anomalies, even if your local logs show success.
• Detailed logging: Implement comprehensive server-side logging that captures the full SMTP conversation, including all command-response pairs. This allows for a deeper dive into why DATA commands might not be fully processed after RCPT acceptance.
• SNDS metric definitions: Rely on Microsoft's official documentation for how SNDS defines and counts RCPT and DATA commands. Their internal definitions may account for situations where messages are processed differently after the initial SMTP handshake. This aligns with a broader need to understand how Postmaster Tools report data.
• Server-side filtering: Be aware that recipient servers can employ complex filtering mechanisms that operate after the initial SMTP commands. A successful RCPT might occur, but the message can still be discarded internally before delivery to the inbox, influencing DATA command counts as reported by SNDS.