As of January 2020, ProtonMail no longer supports older versions of the TLS (Transport Layer Security) protocol: versions 1.0 and 1.1. Going forward, we will only support TLS 1.2 or later.
Why we made this change
Your security is our number one priority, and these protocols are no longer secure. TLS 1.0 and 1.1 are decades-old protocols that create digital signatures using obsolete, insecure hash functions. They also rely on obsolete CBC (cipher block chaining) ciphers, many implementations of which have been found to contain vulnerabilities. Additionally, all major browsers will be dropping support as of early 2020.
For these reasons, and given that TLS 1.2 is both secure and now widely used, we have decided to deprecate the outdated protocols.