It is not possible to create a user for the mailbox:
PLESK_ERROR: The user with the username firstname.lastname@example.org already exists and owns another mailbox.
email@example.com not displayed in Plesk.
Database inconsistency caused by bug PPPM-8022.
Connect to the server via SSH
Delete the user via the command-line interface:
# plesk bin user -d firstname.lastname@example.org