How can security teams tell whether their SEG is still adding value in Microsoft 365?

How to tell whether the SEG is still adding distinct value

For Microsoft 365, the useful test is not whether the SEG blocks email in general, it is whether it still stops something Microsoft’s own controls do not already catch. That usually means looking for behavioral or identity-driven attacks, sender abuse patterns, and other messages that look clean at the payload layer but still create material risk.

A SEG that mostly duplicates malware filtering, URL rewriting, or phishing heuristics already covered natively is not obviously adding control depth. The stronger the microsoft 365 security stack becomes, the more the SEG has to prove unique coverage rather than simply provide another layer that alerts on the same thing twice.

What to measure in Microsoft 365 before you keep paying for a SEG

Start with the threat classes your current stack actually misses. If the SEG is only good at detecting known-bad attachments or links, that is baseline hygiene, not strong evidence of incremental value. What matters is whether it materially reduces exposure from attacks that arrive without a malicious payload, abuse trusted communication paths, or depend on user interaction rather than file detonation.

Teams should also separate prevention value from visibility value. Some SEG products are still worth keeping because they improve traceability, quarantine workflows, or investigation speed, but that is a different claim from blocking unique threats. NIST Cybersecurity Framework 2.0 is helpful here because it forces the question of whether the control strengthens protection, detection, response, or recovery in a way that matters to the operating model.

It also helps to test the SEG against the Microsoft 365 attack surface, not against legacy mail-era assumptions. If the main reason to keep it is “we have always used an SEG,” then the control case is weak. If it provides separate value by catching impersonation patterns, connector abuse, or campaigns that bypass standard content scanning, that is a stronger argument than generic blocking.

Where SEG value usually disappears, and where it can still matter

The value often disappears when the SEG and Microsoft 365 are solving the same problem from the same telemetry. In that case, the SEG may still look active, but it is only repackaging native filtering with another policy layer. That creates cost, administrative overhead, and false confidence without necessarily improving outcomes.

SEG value can still be real when it adds a distinct inspection point, a different threat intel feed, or workflow integration that Microsoft 365 does not provide in your tenant and licensing model. The best external check is whether the SEG catches classes of abuse that are empirically outside your native control set, rather than merely surfacing the same inbox junk under a different brand. FIRST is relevant as a reminder that the operational question is incident handling and coordination, not just message filtering.

For teams already focused on email-driven identity abuse, the more modern test is whether the SEG meaningfully reduces credential theft, account takeover, or session abuse after delivery. That is where MITRE ATT&CK Enterprise Matrix is useful, because it shifts evaluation from “did the message get blocked?” to “did the attack path get broken?”

Practitioner Guidance

What to verify: Compare SEG detections against Microsoft 365 native detections for the same time window and classify the overlap. If most hits are duplicate commodity spam, known phishing, or attachment filtering, the SEG is not proving unique protection.

Decision rule: Keep the SEG only if it consistently adds one of three things: unique threat coverage, materially better investigation workflow, or a measurable reduction in successful compromise. If it does none of those, treat it as a legacy control under review rather than a necessary layer.

What practitioners underestimate: A SEG can look effective even when it is only intercepting noise. The real test is whether it changes outcomes against threats that survive Microsoft’s native filtering and still matter operationally.

Practitioner takeaway: In Microsoft 365, SEG value is earned by unique reduction in risk, not by another layer of mail inspection that mainly duplicates what the platform already does.

Related resources from NHI Mgmt Group

• How should security teams evaluate whether a legacy secure email gateway still adds value in Microsoft 365 or Google Workspace environments?
• How should security teams decide whether to keep a legacy SEG with Microsoft 365?
• How can security teams tell whether their remote access model is still too dependent on perimeter trust?
• How can teams tell whether DSPM is improving Microsoft 365 governance?