- Plesk Onyx for Windows
Mails are delivered locally in case of mail service is disabled for domain and Smartermail server version >= 14.3 is used.
In order to use external mail server, mail service was disabled in Plesk Onyx according to documentation . But if Smartermail of version >= 14.3 is used, emails are still delivered locally because Smartermail has its own settings as Local ones.
This is Plesk bug with ID #PPPM-5200 , that is planned to be fixed in future Plesk updates.
As a temporary workaround, remove or rename SmarterMail domain manually.