Also, more testimony to the same problem (by others) is posted over at ServerFault (StackOverflow):

https://serverfault.com/questions/975871/forcing-dovecot-2-3-4-1-to-use-tlsv...

On 5/8/20 11:50 AM, Steve Egbert wrote:

I have an operational need to disable TLSv1.3 due to inadequate support to exclude certain ciphers.

Much to my dismay, the ssl_protocols had been renamed and re-functionalized into ssl_min_protocol.

Now, there is no way to exclude a specific group of one or more TLS versions.

For a new bug report, I think we need two new settings:

• ssl_tls13_ciphersuite and
• ssl_tls10_cipher

settings introduced into Dovecot for better granularity.

ALong with support for fallback to TLSv1.2 as outlined in https://bugzilla.mozilla.org/show_bug.cgi?id=1250568

I'm still being hammered with the following error with Thunderbird 76.0b3, Dovecot 2.3.4.1-5+deb10u1, Debian 11:

May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL: where=0x10, ret=1: before SSL initialization May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL: where=0x2001, ret=1: before SSL initialization May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL: where=0x2002, ret=-1: before SSL initialization May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL: where=0x2001, ret=1: before SSL initialization May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL alert: where=0x4008, ret=582: fatal protocol version May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL: where=0x2002, ret=-1: error May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL error: SSL_accept() failed: error:14209102:SSL routines:tls_early_post_process_client_hello:unsupported protocol May  8 11:15:47 ns1 dovecot: imap-login: Disconnected (disconnected before auth was ready, waited 0 secs): user=<>, rip=XX.XX.XX.XX, lip=XX.XX.XX.XX, TLS handshaking: SSL_accept() failed: error:14209102:SSL routines:tls_early_post_process_client_hello:unsupported protocol, session= May  8 11:15:47 ns1 dovecot: imap-login: Debug: SSL error: SSL_accept() syscall failed: Invalid argument

This occurred when specifying one TLSv1.3 cipher to be excluded in ssl_cipher via an exclamation mark.

On a side note of IMAP client,  Latest Mozilla Thunderbird had its pref setting security.tls.version.fallback-limit to 4 (TLSv1.3), of which I have adjusted it to 3 (TLSv1.2) and it .... works when Dovecot is set to TLSv1.2.

(Details of Thunderbird security.tls.version.fallback-limit is given in http://kb.mozillazine.org/Security.tls.version.* )

Steve