Mailbox is disabled for mail account (No mailbox) under Domains > example.com > Mail Accounts;
However, mail account receives and stores emails;
The same can be seen over Plesk CLI:
# plesk bin mail -i [email protected]
Go to Extensions, search for Repair Kit extension, and install it.
Go to Tools & Settings > Diagnose & Repair.
Click Repair next to Mail Server:
Connect to the server via SSH.
Restore mail server configuration for the affected domain using the following command:
# plesk repair mail example.com