I think a little more information is needed. What do you mean it can't be like SquirrelMail? I don't get it.
Also why do you not like SquirrelMail, I used it for years and loved it. Sure it was a bit bland out of the box, but a little configuring and tweaking and it became one **** of a webmail application.
HordeMail is another one that is pretty good. Some nice features out of the box, I tried it out but still preferred SquirrelMail so I can't say much on it.