Well, squirrel and horde are the most reliable and secure webmail programs integrated in Cpanel. This is irrefutable plus for them. Can you tell what exactly your clients don't like? Maybe we can help you to satisfy their needs. Also, we can install any webmail program for you in case you are on a VPS or dedicated server, but we can't give guaranty that nothing bad will happen with it since this is a third-party software. Nobody can't foresee the bugs which could take place.