For G Suite.
What should I do when the notification mails of Google Calendar or Google Sites are being sorted as Spam/Junk mails?
We have experienced in the past that such trouble as you have now was settled by having by-path setup in G Suite [spam mail filtering] section.
- Spam mail settings
Add the approved senders on to the list so that the mails will not be sorted as spam mails
Please add the settings for your inside company mails as not to be spam mails.
Then try the settings for addresses of sending notification mails as not to be that of spam mails.