When you have enabled DKIM signing of outgoing mails on your Plesk server, you may have trouble delivering system messages to certain providers. Especially Outlook/Live and ProtonMail can send those messages straight to the spam folder.
Every mail that is not sent using the server’s SMTP but rather the
mail() function is not signed by default. The mail daemon does actually attempt to sign them but can’t find the related account and thus cannot load the domainkey for the intended From address.
The fix is very simple: create a mail account for the user that is sending those mails, most likely
root, on the hostname of the server. So the email address becomes
And make sure DKIM signing is enabled in Domains > servername.tld > Mail Settings.