Maintaining deliverability to Microsoft contacts

Affected contact addresses: outlook, Hotmail and live.com 

Since 2025 and increasingly into 2026, Microsoft has fundamentally strengthened how it filters incoming email, driven by advanced AI-powered systems analysing sender reputation, engagement and trust signals. This shift represents one of the most significant changes to Outlook/Hotmail/Office 365 deliverability in years — and it’s not just a tweak, but a structural overhaul that affects how email is accepted, delivered or outright rejected. 

AI is now central to filtering decisions 

Rather than relying primarily on traditional signature or keyword-based scans, Microsoft’s mail systems now incorporate machine learning and AI models that evaluate each message on multiple dynamic criteria. These models look beyond simple spam markers to assess: 

• Sender reputation  

• Link trustworthiness 

• Real user interaction, such as opens, deletes, and spam complaints 

This AI-centric approach allows Microsoft to better distinguish legitimate mail from increasingly sophisticated unwanted messages, but it also raises the bar significantly for legitimate senders. 

The acceleration of this transition was confirmed in Microsoft’s own documentation and via industry analysis: AI-driven signals are now a core part of bulk sender intelligence and real-time filtering. 

Hard filters replacing soft warnings 

Historically, non-conforming messages were often delivered to Junk folders with warnings as a form of soft enforcement. However, as Microsoft’s AI models have matured, those warnings have been replaced by hard rules: 

• Messages may be rejected at the SMTP level 

• Certain signals (poor reputation, unverified sender authentication, suspected low engagement) can trigger automatic blocking 

• In many cases, there’s no fallback to Junk,  mail simply never reaches the mailbox 

This transition to AI-enforced blocking reflects Microsoft’s confidence in its models’ ability to distinguish harmful from legitimate mail. 

Real engagement matters more than ever 

One of the most consequential changes is how engagement data feeds into Microsoft’s AI scoring. Traditional filters treated user interactions as secondary signals; the new models treat them as core reputation metrics. For example: 

• A high rate of opens and replies now boosts reputation 

• A high rate of deletes without opens or spam complaints lowers reputation 

• Engagement trends over time matter more than just volume or IP history 

This means that AI is learning from how real users interact with mail and making acceptance decisions based on how likely messages are wanted, not just whether they contain known spam traits. 

Why this matters for organisations 

The practical impact of Microsoft’s AI-powered filtering is making a significant and noticeable difference: 

• Messages that once delivered reliably may now be deferred or rejected 

• Traditional blast-and-prune campaigns can trigger reputation penalties 

• Even legitimate business systems can be filtered if sender signals are weak 

Microsoft’s own anti-spam guidance now makes clear that authentication, engagement and trust signals need to align for mail to be accepted, and AI models are the engine making these determinations. 

AI-driven defences for mailboxes too 

It’s not only sender-side filtering that’s changed. Microsoft has also rolled out AI-backed mailbox security features to more users, many of whom may be unaware that these protections are active on their accounts. These defender technologies work in tandem with server-side filters to quarantine or block messages based on nuanced risk assessments from machine learning. 

The future of email filtering 

Microsoft’s embrace of AI for email filtering is a sign of how seriously major providers now take threat detection, user experience and mailbox safety. In practice, this means: 

• Static rules are giving way to adaptive models 

• User signals are weighted more heavily than ever 

• Reputation is dynamic, continuous predictive and no longer static 

For senders and organisations, adapting to this new reality means understanding that AI is now the gatekeeper — and that trust, engagement and authentication are the signals it values most. 

What should you do if you notice Microsoft bounces on your campaign sends? 

Typically, Microsoft bounces will consist of:  

smtp;550 5.7.1 Unfortunately, messages from [0.0.0.0] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [Name=Protocol Filter Agent][AGT=PFA][MxId=11BCD05B3E768AA1] [DS2PEPF00003448.namprd04.prod.outlook.com 2026-02-12T11:34:49.255Z 08DE682DA1DC42A2] 

This means that your sender score/ reputation with Microsoft (e.g. outlook.com, Hotmail.com, Live.com) has dropped, the way in which we can fix this is by temporarily removing the disengaged and slipping away Microsoft contacts. For a minimum of two weeks, once the two weeks have passed you can gradually start to add the Microsoft contacts back into your live campaign sends. This can be done by using a saved filter to remove any disengaged and slipping away contacts on the campaign stage. 

Ensure that when you add the slipping away and disengaged contacts back in, your engaged contacts amount of double of the disengaged and slipping away contacts  

In addition to this, you could also validate your data to ensure that you are sending to valid email addresses and contacts which want to hear from you. 

Find out more about Microsoft using AI to determine how to interpret your emails: 

Microsoft Exchange Spam Filtering Update 2026 Explained | Mailbird 

Microsoft Enforces New Outlook Email Rules in 2026 — What SMBs Must Fix Now 

Changes to Microsoft's deliverability rules in May 2025 

Anti-spam protection FAQ - Microsoft Defender for Office 365 | Microsoft Learn 

What’s changing with Microsoft 365 - and what it means for you - Phoenix Software 

What should you do with your Microsoft disengaged contacts? 

When disengaged contacts present themselves within your CRM there’s different ways which you can, the most popular way is running a disengagement campaign , this means sending to disengaged contact so that they can update their preferences and this leaves your contacts to decide whether they are interested and want to hear from your or if they choose to unsubscribe. 

Learn how to reduce disengagement or run a re-engagement campaign.