As stated in the topic, I would like to install the dovecot imapserver, and enable web-access via the roundcube-client. To heighten security when logging in from unsecure machines, I would like to login via One Time Passwords (e.g. OPIE). Would that be possible? Does anyone have experience implementing such a feature into to a webmail client?
This tutorial is a bit old, but should a good guide whatever two-factor auth system you choose. To get two-factor authentication working on linux, your best bet is to go through PAM. Point PAM to your two-factor auth server via pam-radius, pam-ldap etc.
There's also a feature request to allow SASL OTP in Roundcube.
FYI, there is a One-Time Password plugin for SquirrelMail (OTPs generated in the webmail layer, but can use an optional static prefix deployed via external means).
There is also an authentication plugin that can be configured, amongst other things, to make SquirrelMail SSO-aware (how helpful that is to this question depends on how strong the authentication is for the SSO system).