Why do SNDS RCPT commands not match DATA commands without evidence of bounced emails?
Matthew Whittaker
Co-founder & CTO, Suped
Published 24 Jun 2025
Updated 15 Aug 2025
7 min read
It can be perplexing to see discrepancies between your email sending logs and data reported by services like Microsoft SNDS, particularly when RCPT commands don't align with DATA commands, yet you have no clear evidence of bounced emails. This scenario often leaves senders scratching their heads, wondering where their messages are going if they're not bouncing back.
Understanding this requires a deeper dive into the Simple Mail Transfer Protocol (SMTP) conversation and how different email systems interpret and report these interactions. I've encountered this issue multiple times, and it typically points to subtle delivery nuances rather than outright rejections.
Let's explore why this happens and what it means for your email deliverability. The key often lies in how the receiving server, especially a service like Microsoft Outlook.com, processes emails beyond the initial acceptance of the recipient command.
Understanding SMTP commands and SNDS data
When an email server sends a message, it engages in a series of commands with the recipient's server. Two crucial commands are RCPT TO and DATA. The RCPT TO command specifies the intended recipient of the email. At this stage, the receiving server typically performs initial checks, such as whether the recipient exists or if the sender's IP is on a known blocklist (or blacklist). If the recipient is unknown or the sender is heavily blocked, a hard bounce would usually occur here, resulting in an immediate rejection.
Following a successful RCPT TO, the sender's server sends the DATA command, which transmits the actual content of the email, including headers and body. This is where the bulk of the message is delivered. Once the DATA command is completed, the receiving server accepts the message for further processing, which might include spam filtering, virus scanning, and final delivery to the inbox or spam folder.
SNDS (Sender Network Data Services) provides insights into how Microsoft views your sending reputation and email traffic. It reports on volume and status based on their internal metrics. When SNDS shows more RCPT commands than DATA commands, it indicates that Outlook.com (or other Microsoft properties) accepted the recipient for a higher number of messages than the actual message data they received. This can be confusing if your own logs show a high delivery rate, as you might be asking: What causes delivery errors when no hard bounces are seen?
Understanding SNDS data
SNDS provides high-level insights into your sending reputation with Microsoft, including spam complaints, spam trap hits, and SmartScreen filter assessments. However, it's essential to remember that SNDS data is a view from Microsoft's perspective. It may not always align perfectly with your internal delivery logs, especially when it comes to subtle distinctions like the RCPT/DATA command discrepancy. It helps indicate your overall sender reputation and potential issues that prevent messages from reaching the inbox.
Reasons for RCPT/DATA mismatch
There are several reasons why you might observe more RCPT commands than DATA commands in SNDS without explicit bounce notifications. These scenarios often involve internal filtering or specific SMTP session behaviors.
Multiple recipients in a single SMTP session
One common explanation is when a single email message is sent to multiple recipients within the same SMTP session. Your sending server might issue a single DATA command for the message body, but then issue multiple RCPT TO commands, one for each recipient. In this case, your logs might record one DATA transaction, while SNDS (or the receiving server) counts each individual RCPT TO attempt. This is a normal and expected behavior of the SMTP protocol.
Silent drops and internal filtering
A more concerning reason is when an email is silently dropped. This happens when the receiving server accepts the RCPT TO command, and potentially even the DATA command, but then decides internally to discard the message without generating a Non-Delivery Report (NDR) or bounce notification back to the sender. This can occur due to a variety of factors related to sender reputation, content filtering, or spam trap hits.
If your IP address or domain has poor standing with Microsoft, your emails might be accepted at the RCPT TO stage but then quietly filtered out during the DATA processing or immediately afterward. This leads to the situation where IPs are listed as blocked on SNDS with no Microsoft bounces being reported.
Temporary rejections (soft bounces) without clear NDRs
Another possibility is a form of soft bounce or temporary rejection that doesn't generate a clear NDR you might see in your logs. While servers are supposed to send Non-Delivery Reports, some systems may handle temporary issues in a way that increments the RCPT count without a corresponding DATA acceptance, or without sending an easily digestible bounce notification to the original sender. The sending server might retry, leading to multiple RCPT attempts for a single eventual DATA acceptance, or the message might eventually time out without being fully accepted.
This can make it challenging to diagnose the underlying cause, as your internal systems might only record the final status, which could be delivered if the message eventually went through, or simply not record a hard bounce if the message was quietly dropped. The key is that the RCPT command was accepted, but the DATA part or subsequent processing faced an issue.
Impact on deliverability and troubleshooting
A discrepancy between RCPT and DATA commands, especially without clear bounces, is a red flag for deliverability. It suggests that while your emails are initially accepted, they might not be reaching the inbox as reliably as your logs indicate. This can impact your sender reputation and overall campaign effectiveness.
Sender's logs
Your ESP or internal server logs might show emails as delivered or accepted if the receiving server returns a 250 OK response after the DATA command. This doesn't necessarily mean inbox placement.
Perceived success: Logs may show a high delivery rate, leading to a false sense of security regarding deliverability.
Lack of insight: Without explicit bounces, it's hard to tell if messages are actually reaching the inbox or being silently discarded.
To troubleshoot this, you need to combine your internal logs with external data sources. Review your detailed SMTP transaction logs. Look for any 4xx (temporary failure) or 5xx (permanent failure) responses after the RCPT TO command, even if they don't explicitly show up as bounces in your aggregated reports. Check for subtle differences in the server responses after the DATA command, which might indicate a soft rejection or a spam folder delivery rather than inbox placement.
Also, regularly monitor your sender reputation. If your IP or domain is listed on a blocklist (or blacklist), even a private one, it could lead to silent drops. Services like Suped's blocklist checker can help you identify public listings, but internal reputation scores are harder to gauge directly.
Actively monitor your sender reputation across multiple tools, not just your ESP logs.
Segment your audience and send smaller batches to problematic domains to identify patterns.
Ensure your email authentication (SPF, DKIM, DMARC) is correctly configured.
Maintain a clean mailing list, regularly removing inactive or invalid addresses.
Common pitfalls
Assuming delivery simply because an email didn't hard bounce back.
Relying solely on one data source (e.g., ESP reports or SNDS) for deliverability insights.
Ignoring subtle discrepancies in SMTP command counts, which can indicate hidden issues.
Not considering internal filtering by ISPs that may quietly drop messages.
Expert tips
Pay close attention to the full SMTP conversation, not just the final status code.
Implement feedback loops (FBLs) with major ISPs to receive spam complaint data.
Use seed list testing to get direct inbox placement results across various providers.
If you suspect silent drops, try sending emails with slightly varied content to test filtering.
Expert view
Expert from Email Geeks says RCPT is the number of attempted recipients, and DATA is the number of messages delivered, so if you have one message sent to five recipients, it would be five RCPT but only one DATA.
2020-09-03 - Email Geeks
Expert view
Expert from Email Geeks says RCPT could be higher as a result of a soft bounce or attempted delivery, and DATA was what they ended up accepting, providing two plausible explanations for the observed discrepancy.
2020-09-03 - Email Geeks
Insights into deliverability
The disparity between SNDS RCPT and DATA commands without clear bounce evidence highlights the complexities of email deliverability. It's a strong indicator that you need to investigate beyond surface-level bounce reports.
By understanding SMTP command nuances, closely monitoring your reputation, and scrutinizing your logs for subtle rejections or silent drops, you can gain a more accurate picture of your true inbox placement and take proactive steps to improve your email program.
Microsoft views your sending reputation and email traffic. It reports on volume and status based on their internal metrics. When SNDS shows more RCPT commands than DATA commands, it indicates that 
Outlook.com (or other Microsoft properties) accepted the recipient for a higher number of messages than the actual message data they received. This can be confusing if your own logs show a high delivery rate, as you might be asking: What causes delivery errors when no hard bounces are seen?
