Unverified Sender Visual Marker to Warn of Potential Problems

It’s easy to forget details of the reports about new features that appear in the Microsoft 365 message center, especially when a delay occurs between the expected availability date and when the feature appears in plain sight. This is the case with MC1112452 (10 July 2025, Microsoft 365 roadmap item 491471), which reports the arrival of a new visual indicator in Outlook mobile to highlight messages from unverified senders. The indicator doesn’t mean that the message contains spam or malware; just that its properties are unexpected in a way that might be problematic.

I knew about the feature and had read that mid-July 2025 was the scheduled rollout date. Alas, after that I forgot about unverified visual indicators until a bunch of messages were tagged on August 18 (Figure 1).

MC1112452 says that the new visual warning “aligns with existing functionality in Outlook for desktop and web, bringing a consistent experience across platforms.” Later, the text adds “This feature is available by default when the unverified signal is received by the service.” The support documentation notes “If the message is suspicious but isn’t deemed malicious, the sender will be marked as unverified to notify the receiver that the sender may not be who they appear to be.”

No administrative controls exist to enable or disable the marking of unverified senders. This seems like a pity because external mail tagging, which is another marking applied to inbound messages by Exchange Online, can be turned off or on. The same is true for the first contact safety tip, which can be disabled through the anti-phishing policy actions in the Microsoft Defender portal.

Message Headers and Visual Markers

After examining the headers of several unverified messages, it seems like if the Exchange Online transport service determines that an inbound message can’t be fully verified in terms of the normal tests it applies like SPF, DKIM, and DMARC, the service marks the message as unverified, and users see the warning alongside the message in the message list.

For example, Figure 2 (generated by the Message Header Analyzer) shows that Exchange was unable to check the DKIM status for a message, perhaps because DKIM configuration error in DNS or the email is routed through a third-party mail hygiene system like Mimecast that must be configured in a certain manner for DKIM to work properly.

Same Warnings in other Outlook Clients

As noted above, the functionality now implemented in Outlook mobile matches what users see in Outlook classic, OWA, and the new Outlook for Windows (adjusted to match the UX of the client).

The interesting thing is that Outlook only displays the unverified sender warning in the message list for the first message in a thread. If you reply to an unverified sender, the warning is still present, but only for that message. Any subsequent messages received from that sender are not highlighted, even if the same issues persist with DKIM etc. Perhaps the logic here is that if you engage in an email conversation with a sender, that person is verified by your action and Outlook no longer considers them to be unverified. It’s the best theory that I have to explain what I see happen.

Most Unverified Senders are OK, and Then There’s a Bad One

Marking email coming from unverified senders is a good idea. In many cases, a perfectly simple reason exists for why the verification checks fail and the sending system can quickly adjust their configuration to address the problem. In others, a user could receive a message that’s an attempt to impersonate someone else. Hopefully, messages from attackers will be intercepted by Exchange Online Protection, Microsoft Defender for Office 365, or whatever anti-malware service is deployed, but it’s nice to have another check. Just in case.

Insight like this doesn’t come easily. You’ve got to know the technology and understand how to look behind the scenes. Benefit from the knowledge and experience of the Office 365 for IT Pros team by subscribing to the best eBook covering Office 365 and the wider Microsoft 365 ecosystem.