Update: This blog post is being deprecated and information is being moved to support.office.com: Identify suspicious messages in Outlook.com and Outlook on the web
About a year ago, in Office 365, we released the feature that - similar to Gmail - Outlook Web Access stamps a '?' in the sender photo when the message is not authenticated with either SPF or DKIM.
Well, I was pleasantly surprised to see this morning it's also in Outlook (for Windows, not sure about other clients):
This is great to see, we're now managed to unify the behavior between both Outlook and Outlook Web Access, at least in Office 365.
11/1/2018 - Clarification: this feature is actually still in Preview.