We [Microsoft] are updating how SharePoint Online and OneDrive for Business send external sharing invitations. You’ll begin seeing this change September 2, 2016, with rollout completion expected by the end of the month.

How does this affect me?

Previously, we sent external sharing invitations via the Microsoft Online Services Team address. We have often heard that this e-mail address is confusing and that users have trouble finding invitations that have been sent to them. To address these issues (and to make our external sharing mails consistent with our internal sharing mails), we will now be sending external sharing invitations via the Exchange Web Services. Sharing invitations will now come from a SharePoint owned "No Reply" address.

What do I need to do to prepare for this change?

No changes are required at this time. External users will continue to receive invitations with no change in the acceptance flow.