Roundcube is very good solution for webmail. I use squirrelmail for almost 4 years and I like it too.
In production server I install squirrelmail and roundcube as well for give other user different experience 
using mail and mail2 as prefix and multiple domains need special attention in roundcube.
After search in their forum I like this snippet :
function getDomain() {
$host=$_SERVER['HTTP_HOST'];
$prefix=’mail’;
$arr=explode(‘.’,$host);
if ($arr[0]==$prefix) { unset($arr[0]); $host=implode(‘.’,$arr); }
else { $host=”; }
return $host;
}
plus this line :
$rcmail_config['username_domain'] = getDomain();
put that in config/main.inc.php
for my case where I use mail and mail2 just edit the line :
if ($arr[0]==$prefix) { unset($arr[0]); $host=implode(‘.’,$arr); }
into
if ($arr[0]==$prefix or $arr[0]==’mail2′) {
unset($arr[0]); $host=implode(‘.’,$arr);
}
Working well for me
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