this is possible but you will need root access for respective server. best option is to take a
VPS Hosting and implement this. some of our customers have designed custom webmail interface and you can also purchase certain paid webmail packages online to match interface of yahoo and gmail. not sure if you can get all those functionalities in other webmail packages but at least you can have something that your customers will like more than horde or squirrelmail.
honestly speaking, I don't think that anything would be as good as outlook or Thunderbird but if your customers travel a lot and need webmail only then you have to consider a
VPS Hosting and install a custom webmail package.