Plesk comes with some preconfigured "event handlers", as for example:
Home > Tools & Settings > Event Manager
choose for example "Plesk user failed to log in"
Possible command to trigger an eMail - notification, when this event occurs:
echo "Plesk user failed to log in - pls. see your log - file for details" | mailx -r [email protected] -s "Plesk notification triggered by failed login attempt" [email protected]