In order to be able to connect to POP3 / IMAP without STARTTLS, you need to open the Dovecot configuration file:
sudo nano /etc/dovecot/dovecot.conf
Edit the two parameters listed below:
disable_plaintext_auth=no ssl=yes
To return the parameters of the secure connection to POP3S / IMAPS back:
disable_plaintext_auth=yes ssl=required
After the changes in the Dovecot configuration, you need to restart it:
sudo /etc/init.d/dovecot restart
See also:
How to enable SMTP without SSL on port 25 in iRedMail / Postfix