This article describes how to make sure emails sent from the Calendar E-Mail Extension will not be marked as spam. This only applies if you use the second option ‘Send invitations’ in the ‘E-Mail Settings’ section in the app settings.
If you use the 3rd option ‘Send invitations, check availability and book resources’ this is not necessary, because in this case you use a designated Exchange mailbox.
Users and guests should receive an invite automatically if they are added to the Attendees or Guests column when creating an event in the SharePoint calendar.
If this is not the case, please check your spam filter in Exchange admin center https://outlook.office365.com/ecp, click on Protection, Spam filter, click on Default and add the sender …@mg.sharepointsapiens.com to the ‘Sender allow list’.
You can find the sender address if you click on the app in site content and then on ‘Configure’ on a calendar app. In the Email Settings section you can find the e-mail alias.
If this doesn’t solve it, please also add the following IP address to the IP allow list in connection filters: 126.96.36.199. This is our exclusive dedicated IP address.
The following video describes how to do this: https://technet.microsoft.com/en-us/library/jj200718(v=exchg.150).aspx