Timeout errors?

So I have roundcube working on the same domain at which I am hosting the webpage. (ie, the webpage is at www.foo.com http://www.foo.com, mail.foo.com http://mail.foo.com is working). But so far the other two servers I have tried are timing out: mail.messagingengine.comhttp://mail.messagingengine.comand imap.aol.com http://imap.aol.com. Any suggestions on what I can do to help figure out what is wrong? I am getting connection timed out errors.

Also, what is the format for the 'default_host' parameter in main.inc.php? It says you can provide a list to have a dropdown of mail servers but I can't figure out what the delimeter is...space, semicolon, and comma didn't work. Thanks, Osman