The hidden certificate crisis in healthcare email

Email is used to move lab results, care instructions, billing data, referrals, and scheduling information across an ecosystem of providers, payers, and vendors every day.

In a sample of 803,378 unique outbound email relays analyzed by Paubox, roughly 4% of connections went to servers with unverifiable certificates, including expired or self-signed. That translates into potentially millions of protected health information (PHI)-bearing messages each year sent to endpoints whose identity cannot be fully validated. Cloud email platforms frequently deliver these messages anyway, creating a hidden compliance gap.

How TLS and certificates actually work

TLS is the protocol that encrypts data in transit between email servers. When a healthcare organization sends an email containing PHI, TLS creates an encrypted tunnel that prevents outsiders from reading or altering the message during transmission. However, TLS doesn't work on its own. It depends on digital certificates to prove the identity of the server on the other end.

A digital certificate tells the sending system three things:

• the connection is going to the right server
• the server's identity has been verified by a trusted third party
• the encryption keys being exchanged cannot be tampered with

As Bodipudi explains in research on healthcare email encryption, "Transport Layer Security (TLS) encrypts the email content during transmission between mail servers, ensuring that the data remains secure as it travels across the internet."

When a certificate is expired, self-signed, or broken, trust collapses. If a certificate has expired, it means that the identity cannot be verified. A self-signed certificate is created by the server itself rather than verified by a trusted certificate authority, meaning there is no independent way to confirm the server's identity.

With a bad certificate, encryption can downgrade, fail silently, or get bypassed entirely. As cryptographic researchers, Krawczyk, Paterson, and Wee note in their analysis of the TLS protocol, the security of the entire system "relies crucially on there being no side channel that would reveal the existence of decryption failures to the attacker." When certificate validation fails, the assumptions underlying secure transport collapse, and most organizations never realize it happened.

Read more: Securing healthcare email authentication with DKIM

The silent failure problem

Cloud email platforms usually accept weak or unverifiable certificates because the alternative is message failure. Research cited by Paubox found that many SMTP servers fall back to plaintext or accept invalid certificates when TLS negotiation fails, exposing messages to active interception and downgrade attacks.

Microsoft 365 and Google Workspace don't treat certificate failures in email the same way browsers do. When a certificate fails in a browser, the connection is blocked unless the user deliberately bypasses a warning. Most people recognize this as the security warning page that appears when a website cannot be trusted. With email transport, platforms make exceptions. They negotiate the strongest connection they can, and if they can't verify the certificate, they deliver the message anyway.

The sender sees nothing unusual. Logs show nothing alarming. The recipient gets the message. But the encryption layer is weaker than expected, or in some cases, not guaranteed at all. This is exactly how encryption fails in healthcare. The 2025 Paubox Healthcare Email Security Report found that 60% of healthcare IT leaders reported breaches or security incidents involving email in the past year, yet 74% remain dissatisfied with their current email security solutions. The gap between assumed security and actual security represents one of healthcare's most overlooked vulnerabilities.

Go deeper: Differences between compliance and security

What the data reveals

According to the 2025 Paubox analysis, 16% of email-related breaches involved business associates. A single broken certificate on a billing vendor's server can expose thousands of patient records. Microsoft 365 was involved in 52% of email-related breaches in 2025, up from 43% in 2024, reflecting how certificate-handling decisions by major platforms amplify the risks.

Smith and Abbasi's research on healthcare cybersecurity documents that from 2018 to 2023, there was a 93% growth in large healthcare data breaches, with hacking-related incidents accounting for 77% of all breaches by 2023. In that context, certificate failures represent a silent but significant attack surface.

The vendor ecosystem weakness

Healthcare depends on a complex supply chain of IT vendors, billing companies, EHR add-ons, imaging services, and managed service providers. As Schneller and Abdulsalam note in their research on healthcare supply chains, supply expenses are the second-largest expense category after labor, yet "our review of the health services research literature revealed little attention to supply chains in general, including their policy, management, cost, and impact on outcomes."

This oversight extends to email infrastructure. Many business associates still use old, self-managed mail servers or legacy network appliances. Certificates expire. Renewal automation breaks. Certificate chains go incomplete, and nobody notices.

Healthcare organizations rarely audit the certificate infrastructure of downstream vendors. Yet HIPAA's business associate requirements create shared liability. When a vendor's certificate failure enables a breach, the covered entity faces regulatory exposure alongside the business associate.

Self-signed certificates are problematic because they remove the third-party validation that HIPAA expects for authenticated transport security. As the Paubox certificate report explains, "HIPAA doesn't spell out 'no self-signed certs,' but the Security Rule requires organizations to verify the integrity of the connection. A self-signed certificate cannot provide that verification, which means the encryption cannot be trusted or proven."

The COVID-19 pandemic revealed how fragile healthcare supply chains had become. As Schneller and Abdulsalam observed, "We are experiencing a paradigm shift, in health care and other industries, from a focus on supply chain efficiency toward one of supply chain resiliency and contingency planning." Email infrastructure deserves the same reconsideration.

Cloud platforms make it worse

Because a handful of providers like Google and Microsoft now handle a large share of global business email, their certificate-handling decisions impact millions of domains and amplify the risk. When these platforms accept weak or unverifiable certificates to ensure deliverability, they silently propagate risk across every organization that depends on them.

The 2025 Paubox report found that 83% of healthcare IT leaders say legacy systems disrupt day-to-day operations. Yet even organizations that have migrated to modern cloud platforms inherit the permissive certificate policies those platforms enforce.

Bodipudi's research on email encryption notes a limitation in that "While TLS is effective in preventing interception during transmission, it does not encrypt the email content at rest, meaning it can still be accessed by email service providers." Combined with permissive certificate handling, this creates multiple failure points that compliance frameworks struggle to address.

The DevSecOps principle of "shifting left", addressing security concerns earlier in processes rather than as an afterthought, applies directly here. As Johnson, Smith, and Patel argue, organizations must move beyond reactive security. "The speed at which these workflows operate often comes at the cost of security." Healthcare email infrastructure requires the same proactive stance.

HIPAA compliance implications

HIPAA's Security Rule requires covered entities to implement technical safeguards that protect electronic PHI during transmission. The regulation specifically addresses transmission security, requiring organizations to "implement technical security measures to guard against unauthorized access to electronic protected health information that is being transmitted over an electronic communications network."

Self-signed certificates fail this standard. Without third-party verification of server identity, organizations cannot demonstrate that their transmission security meets HIPAA's integrity requirements. The encryption may exist, but it cannot be proven to regulators.

According to Paubox's 2025 report, email-related HIPAA violations frequently result in penalties exceeding $1 million. To date, OCR has settled or imposed civil money penalties in 152 cases totaling $144.8 million. Smith and Abbasi's research documents how enforcement has intensified, stating, "In 2023, it was estimated that hacking-related incidents accounted for 77% of all healthcare data breaches, in striking contrast to the 49% of data breaches in 2009."

The 2025 Paubox survey found that 86% of healthcare IT leaders worry about their organization's HIPAA compliance status. Nearly 70% estimate that a HIPAA violation tied to email would cost more than $250,000, though actual enforcement actions frequently exceed $1 million.

Learn more: HIPAA compliant email: the definitive guide

How to close the gap

Most healthcare providers assume their email is secure because TLS is enabled, but TLS only works when certificates hold the line. Closing the gap requires moving beyond assumptions to verification.

Paubox Email Suite includes encryption certificate verification as a core feature across all plan tiers. This functionality prevents emails from being sent to domains with expired or self-signed encryption certificates, ensuring PHI is delivered only with proper encryption. When certificate validation fails, Paubox automatically delivers the message as a secure Paubox message rather than transmitting it over an unverifiable connection. This approach eliminates dependency on someone else's infrastructure behaving correctly. Healthcare organizations get encryption they can prove, not just encryption they hope is working. As Bodipudi's research emphasizes, "The effectiveness of encryption technologies largely depends on their usability and acceptance by end-users." Solutions that work transparently, without requiring recipients to navigate portals or manage passwords, achieve higher adoption while maintaining security.

FAQs

What is a self-signed certificate?

A self-signed certificate is a digital certificate created by the server itself rather than issued by a trusted certificate authority. Since no third party has verified the server's identity, there is no independent way to confirm the certificate is legitimate. Self-signed certificates cannot satisfy HIPAA's requirement to verify the integrity of connections transmitting PHI.

What is a certificate chain?

A certificate chain is the sequence of certificates linking a server's certificate back to a trusted certificate authority. Each certificate in the chain vouches for the one below it. When any link in this chain is missing, expired, or improperly configured, the entire chain breaks, and the server's identity cannot be verified.

What is a downgrade attack?

A downgrade attack occurs when an attacker forces a connection to use weaker or no encryption instead of the stronger encryption that both parties could support. When TLS negotiation fails due to certificate problems, some email servers fall back to plaintext transmission, making messages readable to anyone intercepting the traffic.

What is SMTP?

Simple Mail Transfer Protocol (SMTP) is the standard protocol used to send email between servers. When an organization sends an email, the message travels from the sender's mail server to the recipient's mail server via SMTP. TLS encryption is applied during this SMTP transmission to protect the message in transit. When SMTP servers encounter certificate failures, they may fall back to unencrypted transmission or accept invalid certificates to ensure delivery.