How should email security teams prevent unauthorized relay abuse in Microsoft 365 routing configurations?

Security teams should deny relay by default, then explicitly allow only trusted Microsoft 365 tenants that have a documented business need. They should review outbound routing settings, restrict who can modify them, and test that unapproved tenants are blocked. Clear administrative controls matter because a permissive relay path can be abused at scale for spam, even when no customer data is exposed.

Why This Matters for Security Teams

Unauthorized relay abuse is a configuration-risk problem with immediate operational consequences. In Microsoft 365, a relay route that is open too broadly can be turned into a spam channel, which harms reputation, increases abuse handling, and can trigger downstream blocking or throttling. The core control objective is not just to make mail flow work, but to make sure only explicitly trusted tenants can use that path and only for the business flows that justify it.

That is why default-deny routing matters more than convenience. Teams often inherit legacy connectors, shared tenant relationships, or “temporary” allowances that never get cleaned up, and those gaps are exactly what attackers and abusers look for. CISA Secure by Design reinforces the same principle at a broader level: secure defaults should reduce the chance that permissive configuration becomes the normal operating state. In practice, many relay issues are discovered only after outbound abuse or delivery complaints force a review of a path that was assumed to be harmless.

For teams that already manage mail security well, the distinguishing factor is governance: who can create, change, and approve relay exceptions, and whether those exceptions are still needed. If the administrative model is loose, relay abuse becomes a scaling problem rather than a one-off misconfiguration.

How It Works in Practice

Preventing relay abuse in Microsoft 365 routing usually starts with a hard rule: deny relay unless a specific tenant, connector, or route has a documented need and an owner. The practical test is simple, if an unapproved tenant can send through your routing configuration, the control is too open. Teams should review outbound routing settings, connector scopes, accepted domains, and any mail-flow rules that expand trust beyond the intended tenant boundary.

Implementation is strongest when configuration, approval, and monitoring are treated as separate tasks:

Limit who can create or modify connectors and outbound rules.
Allow only named trusted tenants, not broad categories or implicit trust.
Validate that blocked tenants fail closed during testing.
Log and review changes so exceptions are visible after they are approved.

This is also where change control matters. Routing settings often look harmless because they are framed as interoperability, but a permissive path can be abused for bulk mail, phishing, or reputation laundering even when no customer data is directly exposed. NIST AI Risk Management Framework is not the primary lens here, but its governance mindset is useful: control effectiveness depends on defined ownership, traceability, and review, not just on technical presence.

These controls tend to break down when multiple business units share a tenant relationship and no one owns the routing exceptions end to end.

Common Variations and Edge Cases

Tighter relay control often increases operational friction, so teams need to balance business interoperability against abuse risk. The edge cases usually involve partner tenants, mergers, staged migrations, or third-party mail services that appear legitimate but still expand the relay surface. Best practice is evolving toward explicit allowlists with periodic revalidation, rather than long-lived assumptions based on historical trust.

One common mistake is treating “internal” traffic as automatically safe. In routed Microsoft 365 environments, internal-looking traffic can still be forwarded through paths that should never be open to broader tenancy use. Another edge case is partial enforcement, where some routes are blocked while alternate connectors or transport rules still provide a path around the intended restriction. That is why validation must test the whole mail-flow path, not only the obvious connector.

For organizations with frequent tenant-to-tenant communication, the practical challenge is lifecycle management: each exception should expire, be re-approved, or be removed. CIS Controls v8 is a good fit for this kind of operational discipline because it ties account and access governance to repeatable review rather than one-time setup. Teams that do not revalidate allowances regularly usually discover the problem only after abuse, not during design.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

NIST CSF 2.0, CIS Controls v8 and NIST SP 800-53 Rev 5 set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
NIST CSF 2.0	PR.AC — Access Control	Relay routing is governed by who may use and change mail-flow paths.
Recommendation — Restrict routing exceptions and admin changes to approved, least-privilege access.
CIS Controls v8	6 — Access Control Management	Supports controlling allowed relay paths and reviewing who can modify them.
	8 — Audit Log Management	Change and mail-flow logging are needed to detect abused or modified relay routes.
Recommendation — Review and revoke unnecessary connector and rule access on a regular cadence. Enable and review logs for routing changes and suspicious outbound mail patterns.
NIST SP 800-53 Rev 5	AC-3 — Access Enforcement	Relay abuse is prevented by enforcing who can use authorised mail routes.
	AU-2 — Audit Events	Routing changes and blocked attempts should be captured for investigation.
Recommendation — Enforce deny-by-default routing and permit only explicitly approved tenants. Log connector changes and blocked relay attempts for review and response.

Practitioner Guidance

What to prioritise: Treat relay routing as an access-control surface, not just a mail-delivery setting. Start by identifying every tenant, connector, and rule that can forward outbound mail, then separate business-justified exceptions from legacy convenience paths.

What to verify: Confirm that blocked tenants are actually blocked in live testing, not just in documentation. Verify who can change routing, how changes are approved, and whether logging shows both the change and the resulting mail-flow outcome.

Decision rule: If the routing exception cannot be tied to a named business owner and a current need, remove it. If a path must remain open for interoperability, narrow it to the smallest trusted scope and schedule revalidation.

Practitioner takeaway: The safest Microsoft 365 routing posture is the one that assumes relay will be abused unless every exception is explicitly justified, narrowly scoped, and continually rechecked.