If you get the error
"ERR Plaintext authentication disallowed on non-secure (SSL/TLS) connections"
You need to check a couple of settings in your Dovecot configuration.
1. First check in /etc/dovecot/conf.d/10-auth.conf and make sure that
disable_plaintext_auth = no
2. You also need to make sure in /etc/dovecot/conf.d/10-ssl.conf that
ssl = yes
ssl = required
Remember to restart dovecot after making the configuration changes.
service dovecot restart