How does TLS inbound affect email deliverability and sender confidence?
Michael Ko
Co-founder & CEO, Suped
Published 22 Jun 2025
Updated 16 Aug 2025
6 min read
Email is foundational to modern communication, but ensuring its secure and reliable delivery is complex. One critical aspect often overlooked is Transport Layer Security (TLS) for inbound email. While many focus on outbound email practices, understanding how TLS affects inbound email flow is crucial for overall email deliverability and building sender confidence.
TLS encrypts the communication channel between mail servers. For inbound email, this means the connection from a sending server to your receiving mail server is secured, protecting the email's content from eavesdropping and tampering during transit. This security layer is vital for maintaining data privacy and integrity.
What is inbound TLS?
When we talk about inbound TLS, it refers to the percentage of mail received by a recipient's mail server (such as Gmail) that arrived over a TLS-encrypted connection. This metric is typically seen in postmaster tools and reflects the sending domain's commitment to secure communication from the perspective of the receiving domain. A high inbound TLS rate suggests that the sending infrastructure consistently uses encryption when delivering mail.
For email service providers and high-volume senders, maintaining a high inbound TLS rate is a strong indicator of a well-configured and secure sending environment. It demonstrates adherence to best practices for email transport security, which is increasingly becoming a baseline expectation for major mailbox providers.
While email authentication protocols like SPF, DKIM, and DMARC verify the sender's identity, TLS ensures the privacy and integrity of the message during transit. Think of it as protecting the envelope while authentication verifies the sender's signature.
Impact on email deliverability
The direct impact of inbound TLS on email deliverability, in terms of whether an email lands in the inbox or spam folder, is often debated. Major mailbox providers, including Outlook and Yahoo, largely employ opportunistic TLS. This means they will attempt to establish a TLS-encrypted connection, but if it fails, they will typically fall back to sending the email in clear text rather than rejecting it outright.
However, this doesn't mean TLS is irrelevant for deliverability. While an unencrypted email might still be delivered, consistently failing to use TLS can subtly impact your sender reputation. Mailbox providers prioritize security and expect senders to adopt modern security standards. A consistently low inbound TLS rate could signal an outdated or poorly maintained sending infrastructure, potentially leading to increased scrutiny of your emails.
For instance, if your email service provider experiences intermittent TLS negotiation failures, leading to fluctuating inbound TLS rates, this technical issue can hinder reliable delivery. Although the email might not be outright blocked, it creates an unstable sending environment. This is a technical issue that typically falls on the email service provider to resolve, as they are responsible for their SMTP server configuration.
Building sender confidence and trust
While the direct impact on deliverability may be indirect, inbound TLS significantly affects sender confidence and recipient perception. When an email is received without TLS encryption, some email clients, like Gmail, may display a 'broken lock' or similar warning to the recipient. This visual cue immediately decreases trust and makes recipients wary, especially if it happens consistently.
Recipient confidence is paramount for successful email marketing and communication. If recipients perceive your emails as insecure, they are less likely to open them, click on links, or engage with your content. This negative perception can lead to reduced engagement metrics, higher complaint rates, and ultimately, a damaged domain reputation.
Just as a website without a valid SSL/TLS certificate would raise red flags and potentially lose search engine ranking, email without proper encryption can erode recipient trust. It's a fundamental aspect of demonstrating legitimacy and professionalism in your email communications, impacting how your brand is perceived by its audience.
Beyond opportunistic TLS: MTA-STS and DANE
While opportunistic TLS is prevalent, it means that if a secure connection cannot be negotiated, the email will still be sent, but in clear text. This carries inherent security risks, especially for sensitive data. To counter this, advanced mechanisms like MTA-STS (Mail Transfer Agent Strict Transport Security) and DANE (DNS-based Authentication of Named Entities) have emerged.
MTA-STS helps ensure that email servers always use encryption and certificate-based authentication when sending email to your domain. It addresses the vulnerability where an attacker could downgrade a TLS connection to an unencrypted one. DANE, on the other hand, validates the authenticity of the mail server's TLS certificate via DNSSEC, providing an additional layer of trust and preventing man-in-the-middle attacks.
Implementing MTA-STS and DANE is crucial for organizations that send or receive sensitive information, as it moves beyond opportunistic encryption to enforced, verified TLS. While setting these up can be more complex than basic TLS, they offer robust protection and align with the industry's push towards ubiquitous email encryption, ensuring true end-to-end security.
Final thoughts on secure email delivery
Ultimately, a consistently high inbound TLS rate is a marker of a secure and professional sending operation. While an unencrypted email might still reach the inbox, neglecting TLS can subtly erode sender reputation and, more critically, recipient trust. Investing in robust TLS configurations, including configuring TLS on your sending domains, protects your emails from interception and assures your recipients that their communications with you are private and secure.
For email marketers and businesses, prioritizing TLS is not just a technical formality, it's a strategic move to safeguard your brand's image and ensure long-term deliverability success. It helps prevent your domain from being flagged or placed on a blacklist (or blocklist), ensuring your messages consistently reach their intended audience.
Views from the trenches
Best practices
Ensure your email service provider supports and maintains a high TLS encryption rate for all outbound connections, as this directly affects your inbound TLS metric.
Implement MTA-STS and DANE for critical communications to enforce TLS and authenticate the receiving mail server, preventing downgrade attacks.
Regularly monitor your email deliverability metrics, including TLS statistics in Google Postmaster Tools, to identify and address any anomalies.
Choose an email service provider with a strong reputation for security and infrastructure reliability, as technical issues are their responsibility.
Common pitfalls
Relying solely on opportunistic TLS without considering the fallback to clear text, which compromises data security for sensitive information.
Overlooking intermittent TLS negotiation failures, which can lead to fluctuating inbound TLS rates and subtle negative impacts on sender reputation.
Not pushing your email service provider to fix technical issues related to TLS, as they are accountable for the underlying SMTP server configuration.
Ignoring recipient warnings like Gmail's 'broken lock' icon, which erodes sender confidence and negatively impacts user engagement.
Expert tips
For sensitive data, opportunistic TLS isn't sufficient; explore DANE or MTA-STS to ensure authenticated, encrypted sessions.
Most mail transfer agents (MTAs) don't validate the destination server's authenticity with opportunistic TLS, making DANE and MTA-STS critical for trust.
A consistently high inbound TLS rate is a strong signal of a well-maintained sending infrastructure, contributing positively to sender reputation.
While direct deliverability impact is debated, the confidence impact of insecure email is clear: recipients will be wary.
Marketer view
Marketer from Email Geeks says TLS may not directly impact deliverability, but Google's acceptance of non-TLS mail means some messages can still get through.
2022-05-12 - Email Geeks
Marketer view
Marketer from Email Geeks says that technical issues with the SMTP session are the responsibility of the folks who own the SMTP server, and this includes TLS negotiation.
Gmail) that arrived over a TLS-encrypted connection. This metric is typically seen in postmaster tools and reflects the sending domain's commitment to secure communication from the perspective of the receiving domain. A high inbound TLS rate suggests that the sending infrastructure consistently uses encryption when delivering mail.
Outlook and 
Yahoo, largely employ opportunistic TLS. This means they will attempt to establish a TLS-encrypted connection, but if it fails, they will typically fall back to sending the email in clear text rather than rejecting it outright.
Gmail, may display a 'broken lock' or similar warning to the recipient. This visual cue immediately decreases trust and makes recipients wary, especially if it happens consistently.
