While login to Webmail it shows :
Internal Server Error User ‘username’ is over quota
even if the user is not overquota.
when you get the above error then follow the steps :
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root@server@# cd /var/cpanel/overquota
root@server [/var/cpanel/overquota]# rm -f username
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That’s It!
Now try login to webmail you will not find any difficulties.