Summary
The mismatch between SNDS RCPT and DATA commands, without corresponding bounce messages, is a complex issue with numerous contributing factors. These encompass SMTP session management where multiple RCPT commands may exist for a single message, server-side techniques like greylisting and tarpitting which delay or reject connections, aggressive spam filtering silently dropping emails, network or server issues interrupting data transfer, deferred acceptance policies, and general SMTP errors. Microsoft also suggests RCPT counts reflect attempts and soft bounces, not just successful deliveries. These factors often prevent a standard bounce message, leaving a discrepancy in SNDS data.
Key findings
- Multiple Recipients, Single Data: A single email can have multiple RCPT commands for different recipients but only one DATA command.
- Greylisting and Tarpitting: Greylisting and tarpitting intentionally delay or reject connections, increasing RCPT counts without guaranteeing DATA transmission.
- Silent Spam Filtering: Aggressive spam filters may silently discard emails after accepting RCPT but before DATA, without a bounce.
- Network Issues: Network issues and temporary unavailability interrupt connections after RCPT but before DATA.
- Deferred Acceptance: Deferred acceptance postpones data transfer, and if failure occurs before DATA, the count is skewed.
- SMTP Errors: Content filtering, size limits, authentication failures, and reputation problems prevent DATA transmission.
- Attempted Deliveries: RCPT counts reflect attempted deliveries, including soft bounces, not just successful ones.
- Spam Traps: Hitting spam traps increases RCPT commands without corresponding DATA commands, as spam traps accept the recipient but do not process the email further.
- Delayed Responses: Recipient servers use tarpitting techniques to slow down potential spammers, delaying the response to RCPT commands and potentially interrupting the data transfer.
- Connection Timeouts: Connection timeouts occurring after the RCPT command but before the DATA command result in increased RCPT counts without corresponding DATA.
- Silent Discarding: Receiving servers might silently discard emails after accepting the RCPT command but before processing the DATA, for various reasons, including advanced spam filtering.
Key considerations
- Monitor SNDS: Continuously monitor SNDS data to identify and address potential deliverability issues.
- Implement Authentication: Implement email authentication (SPF, DKIM, DMARC) to avoid spam filters and greylisting.
- Manage Reputation: Actively manage sender reputation to avoid being labeled as a spammer and tarpitted.
- Optimize Content: Optimize email content to avoid content filters and size limitations.
- Handle Network Issues: Implement retry mechanisms to handle temporary network or server issues.
- Maintain Clean Lists: Maintain clean and up-to-date email lists to minimize hitting spam traps.
- Comply with SMTP: Comply with SMTP standards and handle deferred acceptance policies.
What email marketers say
10 marketer opinions
The discrepancy between SNDS RCPT and DATA commands without corresponding bounces arises from various factors beyond simple delivery failures. These include SMTP session management where a single message might have multiple RCPT commands for different recipients, greylisting causing retries, aggressive spam filtering silently dropping emails, deferred delivery due to temporary server issues, spam traps accepting recipients without processing the message, tarpitting delaying responses, connection timeouts interrupting data transfer, and silent discarding. These mechanisms lead to increased RCPT counts without corresponding DATA commands or bounce messages.
Key opinions
- Multiple Recipients: A single email campaign sent to multiple recipients will generate multiple RCPT commands within one SMTP session, but only one DATA command for the email's content.
- Greylisting: Recipient servers employing greylisting temporarily reject emails from unknown senders, leading to multiple RCPT attempts before successful DATA transfer.
- Spam Filtering: Aggressive spam filters may accept the RCPT command but silently discard the email content (DATA) without generating a bounce notification.
- Deferred Delivery: Email systems might defer delivery due to peak times or temporary issues, causing the RCPT to be accepted while the DATA transfer is delayed or incomplete.
- Spam Traps: Hitting spam traps increases RCPT commands without corresponding DATA commands, as spam traps accept the recipient but do not process the email further.
- Tarpitting: Recipient servers use tarpitting techniques to slow down potential spammers, delaying the response to RCPT commands and potentially interrupting the data transfer.
- Connection Timeouts: Connection timeouts occurring after the RCPT command but before the DATA command result in increased RCPT counts without corresponding DATA.
- Silent Discarding: Receiving servers might silently discard emails after accepting the RCPT command but before processing the DATA, for various reasons, including advanced spam filtering.
- Delayed Responses: Delayed responses during peak times contribute to mismatched command counts.
Key considerations
- Monitor Reputation: Regularly monitor sender reputation to avoid being greylisted or caught in spam filters.
- Implement Retry Mechanisms: Implement proper retry mechanisms to handle temporary delivery failures and avoid being labeled as a spammer.
- Optimize Sending Volume: Adjust sending volume to avoid overwhelming recipient servers and triggering deferred delivery or tarpitting.
- Content Filtering: Ensure email content adheres to best practices to avoid being flagged by spam filters and silently discarded.
- List Hygiene: Maintain clean and up-to-date email lists to minimize the risk of hitting spam traps.
- Monitor SNDS: Continuously monitor SNDS data to identify and address potential deliverability issues proactively.
- Authenticate Email: Implement email authentication methods (SPF, DKIM, DMARC) to improve deliverability and avoid being flagged as spam.
Marketer view
Email marketer from Mailjet explains that their servers often face situations with delayed responses during peak times. The RCPT command is accepted initially, but the server is unable to complete the data transfer in the appropriate timeframe. This leads to a mismatch.
4 Jul 2024 - Mailjet
Marketer view
Email marketer from Email Marketing Forum shares that some receiving mail servers employ greylisting, which temporarily rejects emails from unknown senders. This results in multiple RCPT attempts before a successful DATA transfer, leading to a mismatch in SNDS data.
5 May 2023 - Email Marketing Forum
What the experts say
3 expert opinions
The disparity between SNDS RCPT and DATA commands, in the absence of explicit bounce notifications, can be attributed to several factors. Microsoft explanations suggest RCPT represents attempted recipients while DATA indicates successfully delivered messages, or that RCPT includes soft bounces. Tarpitting, used to deter spammers, delays responses and can lead to senders abandoning the DATA transfer. Greylisting temporarily rejects emails, requiring retries that increment RCPT counts without always resulting in successful DATA delivery, especially if ultimate delivery fails silently.
Key opinions
- Attempted vs. Delivered: RCPT reflects the number of attempted recipients, whereas DATA indicates the number of messages successfully delivered.
- Soft Bounces: RCPT count may include soft bounces or attempted deliveries that do not result in a final DATA transfer.
- Tarpitting Delays: Tarpitting, designed to slow down spammers, causes delays that might lead senders to give up before sending the DATA, resulting in a mismatch.
- Greylisting Retries: Greylisting's temporary rejection necessitates retries, increasing the RCPT count even if the email ultimately fails to be delivered without a hard bounce.
Key considerations
- Interpret SNDS Data: Understand that SNDS data provides insights into attempted deliveries, not just successful ones, and consider soft bounces and delayed responses when analyzing the data.
- Handle Tarpitting: Implement strategies to handle tarpitting, such as maintaining persistent connections and adjusting sending speeds, to ensure emails are not abandoned prematurely.
- Mitigate Greylisting: Take steps to mitigate the effects of greylisting, such as proper email authentication and building a positive sending reputation, to minimize temporary rejections.
- Monitor Delivery Rates: Monitor delivery rates closely to detect and address any issues that may be causing discrepancies between RCPT and DATA counts.
Expert view
Expert from Email Geeks shares two possible explanations from Microsoft employees: One, RCPT is the number of attempted recipients, and DATA is the number of messages delivered. Two, RCPT could be higher as the result of a soft bounce (or attempted to deliver) and DATA was what they ended up accepting.
26 Sep 2024 - Email Geeks
Expert view
Expert from Word to the Wise explains that tarpitting, a technique used by recipient servers to slow down spammers, can cause RCPT commands to be higher than DATA commands. The server delays the response to the RCPT command, and if the sender gives up before sending the DATA, the counts will differ.
31 Oct 2021 - Word to the Wise
What the documentation says
6 technical articles
Discrepancies between SNDS RCPT and DATA commands, without corresponding bounce messages, arise primarily due to network and server-side behaviors. Initial connection refusals lead to retries that increment RCPT counts, while only successful deliveries increment DATA. Network interruptions after RCPT but before DATA cause divergence. Deferred acceptance policies postpone data transfer, and if a failure occurs before DATA, a mismatch results. Standard SMTP protocols allow multiple RCPT commands before a DATA, and if RCPT commands fail subsequently or the connection breaks, discrepancies occur. Content filtering, size limits, authentication failures, and reputation problems can also cause DATA failures without triggering bounces.
Key findings
- Connection Refusals: Recipient servers might initially refuse connections, leading to retries and increased RCPT counts.
- Network Interruptions: Network issues interrupting the connection between RCPT and DATA commands can cause the counts to diverge.
- Deferred Acceptance: Postfix and other MTAs may implement deferred acceptance policies, delaying DATA transfer and increasing the risk of mismatches.
- SMTP Protocol: The SMTP protocol allows multiple RCPT commands before a DATA, and failures after RCPT increase the disparity.
- SMTP Errors: Content filtering, size limitations, and network problems can prevent DATA transmission even after RCPT success, without generating a bounce.
- Gmail Delivery Errors: Gmail delivery errors, including content, authentication, or reputation issues, can cause DATA failures post-RCPT, without a standard bounce.
Key considerations
- Monitor Network Stability: Ensure network stability to minimize interruptions between RCPT and DATA commands.
- Adhere to SMTP Standards: Comply with SMTP protocol standards to ensure proper email transmission and minimize errors.
- Handle Deferred Acceptance: Implement strategies to handle deferred acceptance policies and minimize potential data loss.
- Optimize Email Content: Optimize email content to avoid triggering content filters or exceeding size limitations.
- Authentication Methods: Properly implement and monitor email authentication methods (SPF, DKIM, DMARC) to improve deliverability and reduce authentication failures.
- Reputation Management: Maintain a positive sending reputation to avoid reputation-based delivery errors.
Technical article
Documentation from Google describes various Gmail delivery errors, including cases where the RCPT command is accepted, but the DATA command fails due to content issues, authentication failures, or reputation problems. These failures might not always result in a standard bounce message.
13 Aug 2023 - Google
Technical article
Documentation from Microsoft explains that RCPT commands can be higher than DATA commands because the recipient server might initially refuse the connection. The sending server will retry delivery, incrementing the RCPT command count for each attempt, but the DATA command will only be incremented upon successful delivery.
16 Aug 2022 - Microsoft
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