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
If you want to enable POP3/IMAP services without STARTTLS for some reason
(again, not recommended), please update below two parameters in Dovecot config
file /etc/dovecot/dovecot.conf
and restart Dovecot service:
/etc/dovecot/dovecot.conf
/usr/local/etc/dovecot/dovecot.conf
disable_plaintext_auth=no
ssl=yes
Again, it's strongly recommended to use only POP3S/IMAPS for better security.
Default and recommended setting configured by iRedMail is:
disable_plaintext_auth=yes
ssl=required
Please comment out below line in Postfix config file /etc/postfix/main.cf
and reload or restart Postfix service:
smtpd_tls_auth_only=yes
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