Which email service providers do not support TLS?
Summary
What email marketers say9Marketer opinions
Email marketer from Stack Overflow explains that you can try to telnet to port 25 on the mail server and issue the EHLO command. If STARTTLS is not listed in the response, the server doesn't support TLS.
Email marketer from MXToolbox explains that their service allows you to enter a domain name and run tests, including SMTP tests that will reveal if the server supports STARTTLS and the TLS versions/ciphers offered.
Email marketer from Reddit suggests using `openssl s_client -starttls smtp -connect example.com:25` in a terminal to check if a mail server supports STARTTLS. If the handshake fails or STARTTLS is not negotiated, then TLS is likely not supported.
Email marketer from Reddit notes that while TLS adoption is widespread, especially among major providers, some smaller or older email services might still lack support. Identifying these requires active testing rather than relying on a static list.
Email marketer from Stack Overflow shares an example of how to use Java code to test for TLS/SSL support on an SMTP server. The code attempts to connect and initiate the STARTTLS handshake.
Email marketer from Email on Acid shares that using TLS encryption is vital for securing email communication, preventing eavesdropping and data tampering during transmission. They note that most reputable ESPs support TLS.
Email marketer from Super User forum mentions that older or unmaintained email servers might not support TLS, but finding a comprehensive list is difficult due to the constantly evolving landscape of email infrastructure. He suggests manually testing domains of interest.
Marketer from Email Geeks shares a simple solution to force a bounce by sending to a made up e-mail address at tiscali.it, as they don’t support TLS, resulting in a delivery failure message: "Connected to IPADDR but STARTTLS is not available, delivery attempt not made. (#5.7.10)"
Email marketer from Reddit shares that not using TLS allows others to read the traffic, so ensuring your email provider uses TLS is very important. They also say to check your providers documentation.
What the experts say2Expert opinions
Expert from Word to the Wise explains that it is increasingly rare to find email service providers that do not support TLS in 2018, however they suggest you could possibly use OpenSSL to test for TLS.
Expert from Word to the Wise explains that you can test for TLS by using openssl from command line `openssl s_client -starttls smtp -connect mail.example.com:25`.
What the documentation says5Technical articles
Documentation from Qualys SSL Labs explains that you can use tools like openssl or specialized online services to check the TLS configuration of an email server by connecting and attempting a STARTTLS handshake. The output reveals supported protocols and ciphers.
Documentation from Nmap describes using the `smtp-starttls.nse` script to detect if an SMTP server supports STARTTLS. It attempts to negotiate TLS and reports the results.
Documentation from OpenSSL describes command line tools which can connect to an SMTP server and test TLS. These will show the cipher suite in use if TLS is supported.
Documentation from IETF details that if a STARTTLS command fails, the client should proceed as if the server does not support TLS. This RFC defines the STARTTLS extension for secure SMTP communication.
Documentation from Microsoft outlines that while Exchange Online requires TLS, some legacy on-premises systems might not support it. They recommend upgrading to support TLS 1.2 or later for security reasons.