I looked in the file system, and there is a folder '/var/vmail/clientdomain.tld/client_email_id/Maildir#' ISPConfig did what it had to do here
So the folder structure has been created. However, sub-folders 'cur', 'new', and 'tmp' are all empty. I guess the mail item hasn't been delivered to the mailbox.
This sort of explains why outlook didn't report a message.