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.)