If you see the error "Invalid maildirsize file" when trying to update your email account password, or change the quota, or if you are just having issues with logging in you should try this:

Log into cPanel, click on File Manager, click on the folder icon next to the mail folder, click on the folder icon next to the domain name, click on the folder icon next to the email user name, then click on the file called maildirsize in that folder (don't click on the icon but the file's name itself). In the upper right hand list of links, click to delete the file. Once you delete it, it will reform for the account automatically and it should then have the correct quota size.