Squirrelmail replacement

[Bob Smith]

Does anybody have a recommendation, or more, for replacing 
Squirrelmail?  Every once in a while my users try to download a large 
file or image, and the 2MB limit from Squirrelmail interferes.  
Sometimes after that, Perl gets a little cranky.

Anyway, if you have a recommendation for a different browser based email 
client, I would appreciate it.  I'm aware of Thunderbird, but for some 
of the users, browser based is the only way to fly.

Thanks,

-Bob