Why cPanel is not allowing to add non-existing email account and if it exists why it is not showing up in list. well, here is the solution for this strange problem.
As the error description implies, the first and obvious reason of this error is, you are attempting to add an existing email account. The second reason is, you attempted to delete this email account earlier which turned into an error. This removed an email account physically, but it left the traces in log and config files. To fix this take following steps.
- Login to your Linux machine via SSH as root user.
- Navigate to the following path, locate and remove email account folder in question.
- Similarly, navigate to the following path, open passwd file with editor of your choice. Remove email account entry from that file and save it.