Just wanted to ask, and can't find this info anywhere. Thanks!
You can add your own text-file stuff at the bottom (like "This message was sent from a secure system and is intended for its recipient only... blah blha blah"). But the "Sent by Roundcube" is inside the PHP code somewhere. If I get a chance tomorrow, I may look this up for you. But all you have to do is start rooting through the code to see where a mail body is constructed, and you should find it. If you have a good text editor, you could search the entire folder for that exact string ;)
Its in the config/main.inc.php