With a fully functioning linux-based email server (running postfix/Dovecot) how can one disable the ability to connect to SMTP/POP3/IMAP with external mail clients (or telnet, etc) while still (and only) using Roundcube webmail (that is, a local email service)?
Roundcube connects to Dovecot IMAP to retrieve its mail; and the server in general needs to communicate to the outside world, so simply shutting off the ports/firewall won't work to still be able to send/receive mail within the webmail application.