Some time we receive the 500 Internal Server Error message while browsing webmail etc.
To resolve above error refer following steps.
root@server [/var/cpanel/overquota]#ll | grep your_cpanel_user_name
Once you found out file for your cPanel user name either remove the file or rename it like cpanel_user_name-back etc.
And now browse your webmail but make sure that you have increased disk space for your domain if its really exceeded.