First you have to inform the imap server you are using.

If it's dovecot, you can use the following:

disable_plaintext_auth = yes

and it will only allow plaintext auth coming from localhost, which is good idea if your webmail is hosted in the same machine.

This way roundcube authenticates (localhost to localhost) without encryption and all other hosts must use TLS otherwise are rejected.

2009/7/13 Carlos Williams <carloswill@gmail.com>
>
> I just configured my IMAP server to use TLS. I did 'not' disable plain
> text login but if the client is configured to check for TLS, it's
> available and the connection is made encrypted rather than plain text.
> My question is how can I force all IMAP connections to my server to
> use TLS rather than plain text from RoundCube?
> _______________________________________________
> List info: http://lists.roundcube.net/users/

can