/etc/postfix/main.cf :

   ## -  Only offer SMTP AUTH when talking over an encrypted connection
   smtpd_tls_auth_only = yes


/usr/local/dovecot/etc/dovecot/conf.d/10-ssl.conf :

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


/usr/local/dovecot/etc/dovecot/conf.d/10-auth.conf :

   # Space separated list of wanted authentication mechanisms:
   #   plain login digest-md5 cram-md5 ntlm rpa apop anonymous gssapi otp skey
   #   gss-spnego
   # NOTE: See also disable_plaintext_auth setting.
   ## auth_mechanisms = plain
   auth_mechanisms = plain login