In the config, note that 'default_host' refers to the IMAP server

config/config.inc.php.sample

@@ -27,7 +27,7 @@ $config = array();
 //       or (Windows): 'sqlite:///C:/full/path/to/sqlite.db'
 $config['db_dsnw'] = 'mysql://roundcube:pass@localhost/roundcubemail';
 
-// The mail host chosen to perform the log-in.
+// The IMAP host chosen to perform the log-in.
 // Leave blank to show a textbox at login, give a list of hosts
 // to display a pulldown menu or set one host as string.
 // To use SSL/TLS connection, enter hostname with prefix ssl:// or tls://

config/defaults.inc.php

@@ -120,7 +120,7 @@ $config['apc_debug'] = false;
 // IMAP
 // ----------------------------------
 
-// The mail host chosen to perform the log-in.
+// The IMAP host chosen to perform the log-in.
 // Leave blank to show a textbox at login, give a list of hosts
 // to display a pulldown menu or set one host as string.
 // To use SSL/TLS connection, enter hostname with prefix ssl:// or tls://