'till now, I only start the -SSL versions of courier imapd and pop3d, so users
have to turn on SSL to be able to connect.
The IMAPd has a setting
to force TLS. With this option set in the non-SSL imapd, there's still no
possibility to log in without encryption.
In the pop3d, I did not find such a setting.
Some web-discussions mentioned POP3_TLS_REQUIRED=1 but that one has no effect
Define "no effect".
The POP3_TLS_REQUIRED setting is in the pop3d-ssl configuration file. After
changing the setting, the POP3 server needs to be restarted.