Running Webmin 1.500 and Dovecot 1.2.8. On the "Networking and Protocols" page, the options for "Accept SSL connections" function backwards. The screen indicates the default is "no" when the default is actually "yes". Selecting the "no" or "default" radio buttons writes the text "ssl = yes" to the dovecot.conf file while selecting "yes" writes "ssl = no".
Thanks - I will fix this in the next Webmin release.