What is the difference between ESMTPS and ESMTPSA in email headers?

Key findings

• ESMTPS: This indicates that the Extended Simple Mail Transfer Protocol (ESMTP) was used with Transport Layer Security (TLS) encryption. It means the connection between the mail servers was secured, protecting the email content in transit from eavesdropping.
• ESMTPSA: This signifies ESMTP with TLS encryption and SMTP Authentication. The 'A' stands for Authentication, meaning the sending client or server not only encrypted the connection but also authenticated itself to the receiving server using credentials like a username and password. This adds a layer of trust, as it confirms the sender's identity to the receiving mail transfer agent (MTA).
• RFC 3848: These transmission types are formally defined in RFC 3848, which registers various ESMTP and LMTP transmission types for use in the 'with' clause of a Received header.
• Trust and legitimacy: While both are good, ESMTPSA is generally considered more trustworthy for the initial hop from the client to the sending MTA because it implies sender authentication. ESMTPS is common for server-to-server transfers.

Key considerations

• Internal vs. external hops: ESMTPSA is often seen for internal handoffs (e.g., from an email client to the user's outgoing mail server), while ESMTPS is typically used for server-to-server transfers between different MTAs across the internet.
• Deliverability impact: Although the presence of these indicators is generally positive, demonstrating secure transport, the specific impact on deliverability is indirect. Strong email authentication standards like SPF, DKIM, and DMARC have a more direct influence on inbox placement.
• Header analysis: Checking these values in the Received header helps in tracing the email's path and verifying if expected security measures were applied at each hop. Anomalies might indicate spoofing or misconfiguration.
• Server configuration: Ensuring your mail servers are correctly configured to use STARTTLS and SMTP AUTH is crucial for robust email security and to avoid potential blacklisting (or blocklisting) for insecure sending practices.

Key opinions

• Header readability: Marketers find email headers complex, but acknowledge their importance for diagnostics. They appreciate tools or explanations that simplify header analysis, like understanding the meaning behind ESMTPS or ESMTPSA.
• Security confidence: There's a general understanding that encryption (TLS) is good for email, and indicators like ESMTPS or ESMTPSA confirm that the message traversed securely. This adds a layer of confidence in the sending process.
• Authentication importance: While ESMTPSA's 'A' for authentication is less frequently discussed in depth by marketers, there is an implicit understanding that authenticated sends are more legitimate. This aligns with broader efforts like DMARC adoption.
• Troubleshooting focus: When an email doesn't land in the inbox, marketers are keen to inspect headers for any red flags, even if they need expert assistance to interpret specific technical strings like these SMTP transport types.

Key considerations

• Simplifying technical details: Marketers benefit from clear, concise explanations of technical header elements to better understand their campaigns' technical performance without needing to become protocol experts.
• Monitoring tools: The desire for easy-to-use tools that can parse and analyze email headers is strong, as it enables faster identification of potential issues indicated by these protocol flags.
• Impact on reputation: Marketers consider how technical configurations, including the proper use of TLS and authentication, contribute to positive sender reputation and avoid being caught in blocklists.
• Avoiding red flags: They are wary of unusual header entries (like a 'Mac-Pro.local' hostname in a cloud environment) that could be indicative of suspicious activity or misconfiguration that might lead to messages being flagged as spam.

Key opinions

• Clear distinction: Experts agree that ESMTPS indicates TLS encryption, while ESMTPSA further specifies that both TLS and SMTP authentication were successfully negotiated. This simple distinction is fundamental for header analysis.
• Trust implications: The 'A' in ESMTPSA implies a higher level of trust for the initial message handoff, as it means the sending client or system authenticated itself to the outgoing mail server. This is a critical factor for preventing unauthorized relaying.
• Operational insights: Observing these types in Received headers helps experts understand the specific security and authentication conditions at each hop an email takes. This allows for pinpointing exactly where a security measure may have failed or succeeded.
• Forged headers: Experts are highly attuned to inconsistencies, such as a local hostname like 'Mac-Pro.local' appearing in a cloud-hosted environment (e.g., AWS), which could indicate a forged header or a misconfigured sender, impacting blocklist status.

Key considerations

• Validation rigor: Trust in automated header analysis tools can vary, with some experts preferring manual inspection of raw source due to potential reordering or annotation biases in automated systems.
• Domain and server mapping: Experts frequently note differences in how major email providers, like Google, stamp their Received headers based on the email type (e.g., Gmail vs. Google Workspace), which requires nuanced understanding for accurate tracing.
• System integrity: The effectiveness of ESMTPSA in providing a more trusted connection hinges on the sending MTA's honesty in reporting its authentication status, a factor that requires continuous vigilance in email security.
• Ongoing education: The complexity and evolving nature of email protocols mean that even seasoned experts benefit from continuous learning and discussions, such as those that clarify the precise meaning of header fields like ESMTPS and ESMTPSA.

Key findings

• Formal definitions: RFC 3848 formally registers and defines the with clause parameters for SMTP and LMTP, including ESMTPS and ESMTPSA. These definitions establish a standard for how MTAs indicate the security and authentication status of a transmission.
• ESMTPS: The documentation specifies that ESMTPS is used when Extended SMTP (ESMTP) is employed over a connection secured by Transport Layer Security (TLS), following a successful STARTTLS negotiation.
• ESMTPSA: ESMTPSA is defined as ESMTP over a TLS-secured connection where SMTP authentication (AUTH) has also been successfully negotiated. This implies that the client or sending server has provided credentials to authenticate its identity.
• Chain of trust: The inclusion of these types in the 'Received' header forms a crucial part of the email's audit trail, allowing receivers to verify the security and authenticity (or lack thereof) of each hop in the mail delivery path.

Key considerations

• Interoperability: Adhering to these documented standards ensures that email systems worldwide can correctly interpret the security context of incoming messages, which is fundamental for global email deliverability.
• Security posture: The presence of ESMTPS or ESMTPSA in headers signals a robust security posture, showing that the transmitting servers are enforcing encryption and, in the case of ESMTPSA, sender authentication. This aligns with modern email security best practices.
• Compliance: For developers and administrators, understanding these RFCs is not just about technical accuracy but also about ensuring their systems comply with established internet standards for email transport, which affects how other mail servers perceive and handle their mail.
• Debugging tool: The formal definitions provide a reliable reference for debugging email routing and delivery issues, allowing administrators to definitively confirm whether TLS and authentication were used as expected at each stage of transmission.