Do I need MDR if I already have Microsoft Defender?
Defender is the detection tool. MDR is the people who read what it produces. Whether you need both depends on who is awake at 3am.
This is the question we are asked most often by clients who have just moved to Business Premium or E5. They now own a capable security stack, and they want to know whether the monitoring service on the proposal is genuinely necessary or a way of adding a line item.
What Defender does well
Defender for Endpoint detects behavior, not just known malware. It will flag credential dumping, suspicious process chains, unusual PowerShell, and lateral movement. It can isolate a device automatically and roll back some changes. Defender for Office 365 catches a large share of phishing before it lands, and Defender for Cloud Apps watches session activity. Tuned properly, this is a strong platform.
What Defender does not do
It does not decide what matters. It produces alerts — hundreds a week in a mid-sized tenant, most of them noise, a few of them the beginning of something. Someone has to look at each one, decide whether it is real, correlate it against what happened on the other twelve machines, and act.
And attacks are inconsiderate about timing. The pattern we see repeatedly is compromise on a Friday evening, quiet reconnaissance over the weekend, and impact on Monday morning. The alert fired at 9pm on Friday. Nobody was looking.
What MDR adds
- Human triage, around the clock, so an alert is assessed within minutes rather than at the start of the next working day.
- Correlation across sources — an endpoint alert, an impossible-travel sign-in and a new inbox rule are three separate signals that only mean something together.
- Containment while the incident is small: isolate the device, disable the account, revoke the sessions.
- A managed SIEM collecting logs from places Defender does not reach, including firewalls, servers and third-party SaaS.
- A written record of what happened, which matters for insurers, auditors and clients who ask.
When you can skip it
If you have security staff covering evenings and weekends, and someone owns the Defender portal as part of their job rather than in addition to it, you may not need a managed service. Below roughly a few hundred staff, that is rare — the cost of the headcount alone exceeds the service.
What we would say
Do not buy MDR instead of configuring Defender. Configure Defender first, so the detections are good, then decide whether you have the people to act on them. Our own service runs on Adlumin and monitors endpoints, servers and Microsoft 365, with the SIEM included and critical incidents notified within 15 minutes.