If you, or your clients, are unable to login to your/their email accounts and the system shows this error message:
Plaintext authentication disallowed on non-secure (SSL/TLS) connections
If you do not want to use SSL/TLS connection to get your email, and to disable SSL/TLS secure connection, do the following:
1. Edit dovecot configuration file: /etc/dovecot.conf using your favorite Linux editor such as vi or pico.
2. Change the value for this directive from:
disable_plaintext_auth = yes
TO:
disable_plaintext_auth = no
3. Restart dovecot
( Note: This is a stupid thing to do.)