We have an account with a VERY large inbox (over 80,000 emails), and when logging into Squirrelmail the main pane would not load. Instead we'd be prompted by the browser to download a right_main.php file. Accounts with smaller inboxes would load fine, so I figured it had something to do with system resources. This is an easy fix. Open up your php.ini file, and increase the memory_limit value to something larger than the default 8M. Restart Apache and try opening your big-ass inbox again. If it still doesn't work, increase memory_limit again. Repeat until your inbox will load, but be careful if you have little RAM. I was able to load my 80,000+ inbox by setting memory_limit = 32M.