In running my periodic Nessus scans, it picked up a few medium severity vulnerabilities against Dovecot. One was “SSL Anonymous Cipher Suites Supported” and the other, “SSL Weak Cipher Suites Supported.”

Look in the Dovecot config file located in /etc/dovecot.conf under “SSL ciphers to use” and you will see:
ssl_cipher_list = ALL:!LOW:!MEDIUM

To disable these weak ciphers change this to:
ssl_cipher_list = ALL:!LOW:!MEDIUM:!MD5:!SSL2:!EXP-ADH-DES-CBC-SHA:!EXP-EDH-RSA-DES-CBC-SHA:!EXP-DES-CBC-SHA:!EXP-EDH-RSA-DES-CBC-SHA:!EXP-ADH-DES-CBC-SHA:!EXP-DES-CBC-SHA:!ADH-AES256-SHA:!ADH-AES128-SHA:!ADH-DES-CBC3-SHA:!EXP-ADH-DES-CBC-SHA:!EXP-ADH-DES-CBC-SHA:!ADH-DES-CBC3-SHA

Run the Nessus scan again and those two vulnerabilities go away