Note that to add multiple mail hosts in main.inc.php you need to use a slightly different format from the default.

Here is the defaul format when installed: $rcmail_configdefault_host? = 'localhost';

And if you want a dropdown list like it's explained in the comments you need something like this:

$rcmail_configdefault_host? = array('mail.example.com', 'webmail.example.com', 'ssl://mail.example.com:993');