What is the default cipher list set to in courier-imap

The Courier documentation describes "TLS_CIPHER_LIST" but doesn't mention a default. A look at the source code reveals that the current default is TLSv1:HIGH:!LOW:!MEDIUM:!EXP:!NULL:!aNULL@STRENGTH. If your version of Courier is built against OpenSSL (in contrast to the packages shipping with Debian/Ubuntu which are built against GnuTLS) you can expand that to a full list of cipher names by running the following command:

$ openssl ciphers 'TLSv1:HIGH:!LOW:!MEDIUM:!EXP:!NULL:!aNULL@STRENGTH'

Example output from an Ubuntu 20.04 machine with OpenSSL 1.1.1f:

TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:ECDHE-PSK-AES256-CBC-SHA384:ECDHE-PSK-AES256-CBC-SHA:RSA-PSK-AES256-CBC-SHA384:DHE-PSK-AES256-CBC-SHA384:ECDHE-PSK-CAMELLIA256-SHA384:RSA-PSK-CAMELLIA256-SHA384:DHE-PSK-CAMELLIA256-SHA384:PSK-AES256-CBC-SHA384:PSK-CAMELLIA256-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-DSS-AES256-GCM-SHA384:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES256-CCM8:ECDHE-ECDSA-AES256-CCM:DHE-RSA-AES256-CCM8:DHE-RSA-AES256-CCM:ECDHE-ECDSA-ARIA256-GCM-SHA384:ECDHE-ARIA256-GCM-SHA384:DHE-DSS-ARIA256-GCM-SHA384:DHE-RSA-ARIA256-GCM-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA256:ECDHE-ECDSA-CAMELLIA256-SHA384:ECDHE-RSA-CAMELLIA256-SHA384:DHE-RSA-CAMELLIA256-SHA256:DHE-DSS-CAMELLIA256-SHA256:DHE-RSA-AES256-SHA:DHE-DSS-AES256-SHA:DHE-RSA-CAMELLIA256-SHA:DHE-DSS-CAMELLIA256-SHA:RSA-PSK-AES256-GCM-SHA384:DHE-PSK-AES256-GCM-SHA384:RSA-PSK-CHACHA20-POLY1305:DHE-PSK-CHACHA20-POLY1305:ECDHE-PSK-CHACHA20-POLY1305:DHE-PSK-AES256-CCM8:DHE-PSK-AES256-CCM:RSA-PSK-ARIA256-GCM-SHA384:DHE-PSK-ARIA256-GCM-SHA384:AES256-GCM-SHA384:AES256-CCM8:AES256-CCM:ARIA256-GCM-SHA384:PSK-AES256-GCM-SHA384:PSK-CHACHA20-POLY1305:PSK-AES256-CCM8:PSK-AES256-CCM:PSK-ARIA256-GCM-SHA384:AES256-SHA256:CAMELLIA256-SHA256:SRP-DSS-AES-256-CBC-SHA:SRP-RSA-AES-256-CBC-SHA:SRP-AES-256-CBC-SHA:RSA-PSK-AES256-CBC-SHA:DHE-PSK-AES256-CBC-SHA:AES256-SHA:CAMELLIA256-SHA:PSK-AES256-CBC-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES128-SHA:ECDHE-PSK-AES128-CBC-SHA256:ECDHE-PSK-AES128-CBC-SHA:RSA-PSK-AES128-CBC-SHA256:DHE-PSK-AES128-CBC-SHA256:ECDHE-PSK-CAMELLIA128-SHA256:RSA-PSK-CAMELLIA128-SHA256:DHE-PSK-CAMELLIA128-SHA256:PSK-AES128-CBC-SHA256:PSK-CAMELLIA128-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:DHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-CCM8:ECDHE-ECDSA-AES128-CCM:DHE-RSA-AES128-CCM8:DHE-RSA-AES128-CCM:ECDHE-ECDSA-ARIA128-GCM-SHA256:ECDHE-ARIA128-GCM-SHA256:DHE-DSS-ARIA128-GCM-SHA256:DHE-RSA-ARIA128-GCM-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA256:DHE-DSS-AES128-SHA256:ECDHE-ECDSA-CAMELLIA128-SHA256:ECDHE-RSA-CAMELLIA128-SHA256:DHE-RSA-CAMELLIA128-SHA256:DHE-DSS-CAMELLIA128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA:DHE-RSA-CAMELLIA128-SHA:DHE-DSS-CAMELLIA128-SHA:RSA-PSK-AES128-GCM-SHA256:DHE-PSK-AES128-GCM-SHA256:DHE-PSK-AES128-CCM8:DHE-PSK-AES128-CCM:RSA-PSK-ARIA128-GCM-SHA256:DHE-PSK-ARIA128-GCM-SHA256:AES128-GCM-SHA256:AES128-CCM8:AES128-CCM:ARIA128-GCM-SHA256:PSK-AES128-GCM-SHA256:PSK-AES128-CCM8:PSK-AES128-CCM:PSK-ARIA128-GCM-SHA256:AES128-SHA256:CAMELLIA128-SHA256:SRP-DSS-AES-128-CBC-SHA:SRP-RSA-AES-128-CBC-SHA:SRP-AES-128-CBC-SHA:RSA-PSK-AES128-CBC-SHA:DHE-PSK-AES128-CBC-SHA:AES128-SHA:CAMELLIA128-SHA:PSK-AES128-CBC-SHA

Note that this list depends on your OpenSSL version so make sure you run this command on your mail server machine. It's a good idea to look through this list carefully. Even on my up-to-date system this list contains quite a few ciphers that have to be considered weak.

https://www.courier-mta.org/couriertls.html sais:

TLS_CIPHER_LIST=cipherlist

• Optionally set the list of protocol ciphers to be used. See OpenSSL's documentation for more information.

So I think it takes the defined ciphers in openssl then.

Note: apart from the Ciphers list, you probably want to set

TLS_PROTOCOL="TLSv1.2"

This will turn off TLS 1.0 and TLS 1.1

Whether TLSv1.2 works depends on whether your openssl library is recent enough to support TLS1.2

In case you want to disable TLS v1.0 and TLS v1.1 in your SSL certs, you have to set it in your webbrowser, e.g. for nginx, see this link: https://libre-software.net/tls-nginx/#tls