It seems that Microsoft and Google continue to tease each other…

As I discovered recently, if you share a file from Google Drive with someone that uses Office 365, the notification email will go directly into the Junk folder.

Solution(s): 
– adding the respective domain on the Office 365 whitelist (ex. @domain-that-is-hosted-on-google-apps.com) 
– creating a contact on your Outlook Online with the senders address (ex. user@domain-that-is-hosted-on-google-apps.com)