Why do SNDS RCPT commands not match DATA commands without evidence of bounced emails?
Summary
What email marketers say10Marketer opinions
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.
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.
Email marketer from StackExchange mentions that hitting spam traps might increase RCPT commands without corresponding DATA commands. Spam traps accept the recipient but don't process the email, and also don't send a bounce message.
Email marketer from Twitter shares that, in some cases, receiving servers might silently discard emails after accepting the RCPT command but before processing the DATA command, without sending a bounce message. This can be due to a variety of reasons, including spam filtering.
Marketer from Email Geeks explains that you can have more than one RCPT in an SMTP session to facilitate the same email going to multiple recipients.
Email marketer from Quora explains that if a connection times out after the RCPT command but before the DATA command, the RCPT count increases without a corresponding DATA command. This scenario doesn't always generate a bounce message.
Email marketer from LinkedIn comments that recipient servers use tarpitting techniques to slow down potential spammers. This involves delaying the response to RCPT commands, which could lead to mismatches if the connection is terminated before the DATA command.
Email marketer from Reddit shares that aggressive spam filtering on the recipient's side might accept the RCPT but silently drop the email content (DATA) without sending a bounce message. This leads to a higher RCPT count.
Email marketer from Email Deliverability Forum explains that temporary server issues at the recipient's end might cause an initial RCPT acceptance followed by a failure before the DATA command. This occurs without generating a standard bounce message.
Email marketer from Email Marketing Blog explains that some email systems defer delivery during peak times or due to temporary issues. The RCPT command is accepted, but the DATA transfer is delayed and might not complete immediately, causing a discrepancy.
What the experts say3Expert opinions
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.
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.
Expert from Spam Resource explains that greylisting, where a mail server temporarily rejects emails from unknown senders, can increase the RCPT count in SNDS without a corresponding increase in DATA. The sending server retries, incrementing the RCPT count, but if delivery ultimately fails without a hard bounce, the discrepancy remains visible in SNDS.
What the documentation says6Technical articles
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.
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.
Documentation from MXToolbox explains common SMTP errors, including situations where an RCPT command succeeds initially, but subsequent DATA transmission fails due to various issues like content filtering, size limitations, or network problems.
Documentation from RFC explains that according to SMTP protocol standards, multiple RCPT commands can precede a single DATA command. Discrepancies arise if any RCPT command fails after being issued or if the connection is interrupted.
Documentation from Cisco describes that network issues or temporary unavailability of the recipient server can cause multiple RCPT attempts. If the connection is interrupted after the RCPT command but before the DATA command, the counts will diverge.
Documentation from Postfix.org explains that Postfix and other MTAs may implement "deferred acceptance" policies to manage load. The RCPT is accepted initially, but actual data transfer is postponed, potentially leading to mismatches if the connection fails before the DATA is sent.