Just two places, first the config file has a way of checking multiple IMAP servers, however it is not documented well.
$rcmail_config['default_host'][] = 'localhost';
$rcmail_config['default_host'][] = 'imap.somedomain.com';
$rcmail_config['default_host'][] = 'imap.anotherdomain.com';
What that will do is give the user a drop down to select when logging in. However, if your going to jump dump it on your SME, just put in the following default setting:
$rcmail_config['default_host'] = 'localhost';
The other config I set, which to be honest I'm not 100% sure I had to, was the default IMAP port:
$rcmail_config['default_port'] = 993;
I know on startup I can see both IMAP and IMAPS starting, so just to be safe I configured roundcube to use 993 instead of 143. However, since its on localhost, I guess using 143 wouldn't hurt.
This is a really slick client, its impressive how far its come in just a few short months (since Alpha).
Geoffrey