Suped

Summary

SSL/TLS key size errors in email SMTP transactions occur when the security keys used by either the sending or receiving server during the Transport Layer Security (TLS) handshake are considered too weak or incompatible by the other party. This issue primarily impacts the secure establishment of the SMTP connection, preventing email delivery.

What email marketers say

Email marketers often encounter SSL/TLS key size errors indirectly, typically as delivery failures or unexplained bounces, rather than explicit technical warnings. Their focus is generally on ensuring messages reach the inbox, making these underlying technical issues a significant, albeit often hidden, impediment to their campaigns.

Marketer view

Email marketer from Email Geeks indicates they are currently using a 1024-bit key and are investigating if this is the source of the problem, particularly if another server expects a larger key size for a secure connection.

19 May 2017 - Email Geeks

Marketer view

A marketer from Quora highlights that an SSL certificate error often indicates issues like an incorrect system date, a domain name mismatching the certificate, or the certificate being self-issued rather than from a trusted authority, all of which can affect secure email transmission.

22 Jun 2023 - Quora

What the experts say

Email deliverability experts highlight that SSL/TLS key size errors are a fundamental issue of cryptographic strength and server compatibility. They emphasize that such errors prevent the secure delivery of emails, often pointing to outdated server configurations or non-compliant certificates as primary causes. Proper configuration is essential for modern email ecosystems.

Expert view

Expert from Email Geeks explains that the error message is specifically related to SSL/TLS, not DKIM, clarifying that it concerns the encryption handshake between the sending and receiving servers during the SMTP transaction.

19 May 2017 - Email Geeks

Expert view

An expert from SpamResource states that weak or unsupported cipher suites and outdated SSL/TLS versions are common culprits for handshake failures, urging senders to update their server configurations to meet modern security expectations.

15 Feb 2024 - SpamResource

What the documentation says

Technical documentation consistently emphasizes that strong cryptographic key sizes are fundamental for secure communication, including SMTP transactions. As standards evolve, older, weaker key sizes are actively phased out, and systems are designed to reject connections that do not meet current security benchmarks. This ensures forward secrecy and protects data integrity during email transfer.

Technical article

Sectigo Official documentation notes that certificate issues, including revoked, inactive, or expired certificates, are common causes of TLS errors, implying that key size, though not explicitly mentioned here, is a critical component of certificate validity.

27 Apr 2021 - Sectigo Official

Technical article

The IETF Datatracker, via RFC 8446 for TLS 1.3, specifies robust cryptographic requirements for establishing secure communication over the Internet, effectively making older, weaker key sizes non-compliant with the latest protocol version.

10 Aug 2018 - IETF Datatracker

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