SquirrelMail is a completely different kind of product…

Port87 is a hosted email service. SquirrelMail is a self-hostable webmail client.

SquirrelMail also predates AJAX and nearly predates JavaScript itself. It came out 24 years before Port87. The latest official release of SquirrelMail is also 13 years old at this point. You should be using Roundcube if you want self hosted webmail (which also requires JavaScript).