If package is allowed zero mailboxes, disable mail service for domain.
We only use Plesk for webhosting and use our own system for e-mail.
However Plesk has the nasty habit of enabling mail service for the domain by default even if you give the package zero mailboxes.
With the result that if a PHP web application sends e-mail, the Plesk server thinks it is responsible for handling the e-mail, and will bounce it because the user does not exist locally.
Even more stupid, the option to disable mail handling for the domain manually is not accessible either when a package has zero mailboxes.
If you use external mail system, you can either disable mail services for the whole Plesk server in Tools & Settings → Services Management. Either way you can disable mail for selected domains included into specific service plan in Service Plan → Mail.