You're getting a HTTP internal error 500 and finds this message in the error log:
- Premature end of script headers: index.php
This happens when Apache is configured to execute your *.php files as cgi-bin scripts.
Remove the ScriptAlias directive from your Apache configuration file.
If you have scripts you need executed, then you may just want to move the main squirrelmail directory somewhere else under DocumentRoot. Just make sure you keep it out of the directories listed under ScriptAlias.