Starting now (and scheduled to be completed by the end of January), Microsoft is updating the FROM address for email sent by SharePoint Online.
The new email address will be no-reply@sharepointonline.com. This change is to address issues with such emails which could frequently being marked as spam and so either received in the Junk folder or even being dismissed depending of the anti-spam configuration.
This means also that the email address you have defined in SharePoint Online to send notification will no longer be used and it will be removed.
