If you are getting the following error when while you access your webmail
Query: FETCH 1 (FLAGS UID RFC822.SIZE INTERNALDATE BODY.PEEK[HEADER.FIELDS (Date To Cc From Subject X-Priority Importance Priority Content-Type)])
then probably you are having permission issues with the mail directory of your emails.
chmod to mail directory and change the mail folder to user user.
chown user.user /path/to/maildir -R
for cPanel emails it is chown username.username
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