You used to sort of be able to do this - at least you could "select all" and then uncheck individual addresses... they may have removed this because it's a feature that can so easily be abused to spam your whole addressbook.
A plugin like this:
which lets you create different "groups" of addresses might work for some applications....
Also, if anyone is so motivated, the developers of Squirrelmail are usually responsive to feature requests, and so if there are areas where the UI or functionality could be improved, please feel free to suggest them.
All webmail systems that I've seen are deficient in some way - Squirrelmail at least makes it fairly easy for people to extend the functionality via third party plugins, and has a pretty responsive developer community.
Also, because of the way our system is setup, we have to do some hooks to make "email@example.com" login to webmail and get to the right place. While we're not open source bigots, doing this with a proprietary / commercial webmail system would likely be much trickier in many cases (though we might be able to create a simple login program as a gateway for such a tool).
We have been working on the overloading and general issues with webmail; we believe we've at least partially fixed a tricky internal networking problem that might have been responsible for some of the slowdown; we've made some other improvements that should help, and are looking into adding some more webmail machines. Things seem to have cleared up / calmed down quite a bit today.
Thread drift - I haven't used it yet, but I hear that Gmail has a great interface and text editor... how do people who have tried it like it? Now if only they'd open source their webmail system....