This can happen when switching the domain name on an account:
Sorry, you do not have access to the domain domain.com
Remove the following folders, if they exist:
/home/user/mail/olddomain.com/xxx
/home/etc/olddomain.com
Locate and backup the following files:
/home/user/.cpanel/
email_accounts.yaml
email_accounts.cache
Empty email_accounts.cache and edit email_accounts.yaml to remove the accounts/domain no longer required.
If it still does not work, you can try running ‘/scripts/whoowns olddomain.com’ for some additional information.