I finally got my hands on (a now somewhat dated) issue 6.04 of PHP Magazine, which included an article about IlohaMail by David Costa. Here are some choice quotes:

I was disappointed by the performance of Squirrelmail under PHP 5. In fact, I was unable to successfully run any version prior to 1.4.3a on PHP 5, and even 1.4.3a didn't have the stunning installation simplicity and speed provided by IlohaMail.

Peek into the IlohaMail source code and you will notice that this application has been well developed and well commented. Much of the credit goes to the lead developer Ryo Chijiiwa. This is certainly an important aspect: There are thousands of PHP free applications online, but not many are well coded and therefore easy to expand, customize, and maintain.

IlohaMail is one of the best webmail solutions available. It's strength resides in its ease of installation, minimal requirements, flexibility, speed, and excellent code.