Security teams should treat external mail forwarding as an egress control, not just a mailbox preference. The rule should be inventoried, tied to an owner, justified for business use, and reviewed alongside access changes and offboarding. If a rule copies messages outside the tenant without governance, it creates a durable exposure path that can outlive the original access decision.
Why This Matters for Security Teams
External mail forwarding is often treated as a convenience setting, but it behaves like an outbound data path with persistence, not a harmless inbox preference. Once mail leaves the tenant, mailbox controls, retention policies, and many internal review processes no longer apply in the same way. That makes forwarding rules especially relevant to phishing, business email compromise, insider misuse, and accidental exfiltration of regulated data. The NIST Cybersecurity Framework 2.0 emphasizes asset visibility and protective controls, and the same logic applies here: a forwarding rule is an email-control asset that must be governed like any other egress mechanism.
Security teams also need to distinguish legitimate business workflows from unmanaged persistence. A forwarding rule can survive password resets, user confusion, and in some cases weak offboarding if it is not actively removed or monitored. NHIMG’s Top 10 NHI Issues highlights how durable credentials and uncontrolled access paths create long-lived exposure, and forwarding rules create a similar governance problem for mail flow. In practice, many security teams encounter external forwarding only after sensitive messages have already been redirected outside the tenant, rather than through intentional review.
How It Works in Practice
Effective governance starts with inventory and classification. Security teams should identify every rule that forwards messages outside the organization, then assign an owner, business purpose, review date, and approval record. Rules that support travel, executive assistants, legal counsel, or partner workflows may be valid, but they should still be explicit, time-bounded, and monitored. If the platform allows it, external forwarding should be centrally restricted and exceptions should require documented justification.
Practical control points usually include:
- Disable open-ended user-created external forwarding by default.
- Allow only approved domains or partners where business need is clear.
- Require managerial or security approval for exceptions.
- Review forwarding rules during joiner-mover-leaver events and periodic access recertification.
- Alert on new external rules, destination changes, and rule creation after a mailbox compromise.
Governance is stronger when forwarding review is tied to offboarding and access change workflows rather than handled as a separate mailbox cleanup task. That matters because mailbox rules are often invisible to identity teams until a complaint, audit, or incident exposes them. NHIMG’s Ultimate Guide to NHIs — Lifecycle Processes for Managing NHIs is written for NHI lifecycle discipline, but the operational lesson is transferable: durable access paths must be tracked from creation to retirement. These controls tend to break down when organizations allow user-managed forwarding in hybrid mail environments because policy enforcement becomes fragmented across cloud, on-premises, and legacy transport rules.
Common Variations and Edge Cases
Tighter forwarding control often increases helpdesk and business-operations overhead, so teams have to balance exfiltration risk against legitimate productivity needs. There is no universal standard for this yet, but current guidance suggests external forwarding should be prohibited by default unless the business case is concrete and reviewable. Some organizations allow selective forwarding only to trusted vendors, while others permit it only through transport rules created by administrators. The right choice depends on regulatory exposure, mailbox sensitivity, and how much monitoring the SOC can sustain.
Edge cases matter. Shared mailboxes, delegated assistants, mergers, and legal hold scenarios may require carefully scoped forwarding, but those exceptions should not become permanent policy gaps. Security teams should also watch for attackers who add forwarding after compromising a mailbox, because the rule becomes a low-noise persistence mechanism. NHIMG’s DeepSeek breach illustrates how exposed access paths and sensitive data persistence can create broad downstream impact once controls are lost. For broader context on secrets and abuse patterns, the NHIMG article The State of Secrets in AppSec is useful, especially where mailbox forwarding intersects with credential theft. In environments with heavy automation, mail routing, or third-party journaling, this guidance breaks down when rule ownership is unclear and no team can prove who approved the outbound path.
