With default iRedMail setting, all clients are forced to use POP3/IMAP/SMTP services over STARTTLS for secure connections. If your mail clients try to access mailbox via protocol POP3/IMAP without TLS support, you will get error message like below:
Plaintext authentication disallowed on non-secure (SSL/TLS) connections
This tutorial describes how to allow insecure connection for daily use.
If you just have 1 or few network devices like printer, firewall need to send email with insecure connection, please follow this tutorial instead: Allow internal network devices to send email with insecure connection.
If you want to enable POP3/IMAP services without STARTTLS for some reason
(again, not recommended), please update below two parameters in Dovecot config
/etc/dovecot/dovecot.conf and restart Dovecot service:
Again, it's strongly recommended to use only POP3S/IMAPS for better security.
Default and recommended setting configured by iRedMail is:
Please comment out lines below in Postfix config file
and reload or restart Postfix service: