Secure Dovecot SSL settings by following Mozilla's Security/Server Side TLS guidelines

15-ssl-intermediate.conf

##
## Dovecot SSL settings with Intermediate compatibility
## Follows Mozilla's Security/Server Side TLS guidelines
## https://wiki.mozilla.org/Security/Server_Side_TLS
##

##
## Optionial:
##   Disable 3DES ciphersuites to prevent CVE-2016-2183
##     by appending ":!3DES" to the ssl_cipher_list
##   Disable TLSv1 for PCI compliance as of June 30, 2018
##     by appending " !TLSv1" to the ssl_protocols for versions < 2.3
##     by changing ssl_min_protocol to "TLSv1.1" for versions >= 2.3
##

# SSL/TLS support: yes, no, required. <doc/wiki/SSL.txt>
ssl = required

# ciphersuites
ssl_cipher_list = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA:!DSS

# versions
# for dovecot < 2.3
#ssl_protocols = !SSLv3 !SSLv2
# for dovecot >= 2.3
ssl_min_protocol = TLSv1


# DH parameter size
# for dovecot < 2.3
#ssl_dh_parameters_length = 2048
# for dovecot >= 2.3
ssl_dh = <ffdhe2048.pem
# Get ffdhe2048.pem from https://wiki.mozilla.org/Security/Server_Side_TLS#Pre-defined_DHE_groups

# Prefer the server's order of ciphers over client's.
ssl_prefer_server_ciphers = yes

# SSL extra options. Currently supported options are:
#   no_compression - Disable compression.
#   no_ticket - Disable SSL session tickets.
ssl_options = no_compression no_ticket

15-ssl-modern.conf

##
## Dovecot SSL settings with Modern compatibility
## Follows Mozilla's Security/Server Side TLS guidelines
## https://wiki.mozilla.org/Security/Server_Side_TLS
##

## Oldest compatible client
## iOS 10, gmail App ?, macOS Sierra 10.12, Outlook 2013 on Windows 8.1
## Samsung Mail ?, Andorid 5, Java 8

# SSL/TLS support: yes, no, required. <doc/wiki/SSL.txt>
ssl = required

# ciphersuites
ssl_cipher_list = ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256

# versions
# for dovecot < 2.3
#ssl_protocols = TLSv1.2
# for dovecot >= 2.3
ssl_min_protocol = TLSv1.2

# DH parameter size
# not technically needed since no DHE cipher suites are enabled
# but dovecot >= 2.3 will throw a warning when ssl_dh is not set
ssl_dh = <ffdhe2048.pem
# Get ffdhe2048.pem from https://wiki.mozilla.org/Security/Server_Side_TLS#Pre-defined_DHE_groups

# Prefer the server's order of ciphers over client's.
ssl_prefer_server_ciphers = yes

# SSL extra options. Currently supported options are:
#   no_compression - Disable compression.
#   no_ticket - Disable SSL session tickets.
ssl_options = no_compression no_ticket

15-ssl-old.conf

##
## Dovecot SSL settings with Old backward compatibility
## Follows Mozilla's Security/Server Side TLS guidelines
## https://wiki.mozilla.org/Security/Server_Side_TLS
##

# SSL/TLS support: yes, no, required. <doc/wiki/SSL.txt>
ssl = required

# ciphersuites
ssl_cipher_list = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:ECDHE-RSA-DES-CBC3-SHA:ECDHE-ECDSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:DES-CBC3-SHA:HIGH:SEED:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!RSAPSK:!aDH:!aECDH:!EDH-DSS-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA:!SRP

# versions
# for dovecot < 2.3
#ssl_protocols = !SSLv2
# for dovecot >= 2.3
ssl_min_protocol = SSLv3

# DH parameter size
# When dovecot < 2.3 starts up for the first time, it generates new 512bit and 1024bit Diffie Hellman parameters and saves them
#ssl_dh_parameters_length = 1024
# for dovecot >= 2.3
ssl_dh = <dh.pem
# generate dh.pem with openssl dhparam -C 1024 > dh.pem

# Prefer the server's order of ciphers over client's.
ssl_prefer_server_ciphers = yes

# SSL extra options. Currently supported options are:
#   no_compression - Disable compression.
#   no_ticket - Disable SSL session tickets.
ssl_options = no_compression no_ticket

Note, Dovecot 2.3 has improved defaults. Notably, SSLv2 is no longer possible and SSLv3 is disabled by default. Additionally, DH params are no longer generated and must be provided.